From:  http://www.ietf.org/internet-drafts/draft-ietf-calsch-many-xcal-00.txt
See: http://xml.coverpages.org/iCal.html
     iCalendar DTD Document (xCal)

4. iCalendar XML Document Type Definition

   The following DTD conforms to XML version 1.0, as specified by
   [XML]. 

   <?xml version="1.0" encoding="UTF-8"?>

   <!-- ******************* -->
   <!-- Entity declarations -->
   <!-- ******************* -->

   <!ENTITY % attr.altrep "
   altrep ENTITY #IMPLIED
   ">

   <!ENTITY % attr.cn "
   cn CDATA ''
   ">

   <!ENTITY % attr.cutype "
   cutype NMTOKEN 'INDIVIDUAL'
   ">
   <!-- Valid name tokens are "INDIVIDUAL", "GROUP", "RESOURCE" -->
   <!-- "ROOM", "UNKNOWN", a non-standard "X-" name or another -->
   <!-- IANA registered name. -->

   <!ENTITY % attr.delegated-from "
   delegated-from CDATA #IMPLIED
   ">
   <!-- delegated-from value is a calendar user address -->

   <!ENTITY % attr.delegated-to "
   delegated-to CDATA #IMPLIED
   ">
   <!-- delegated-to value is one or more calendar user addresses -->

   <!ENTITY % attr.dir "
   dir ENTITY #IMPLIED
   ">
   <!-- dir value is a URI to a directory entry -->

   <!ENTITY % attr.fmttype "
   fmttype CDATA #REQUIRED
   ">
   <!-- fmttype value is any IANA registered content type -->

   <!ENTITY % attr.fbtype "
   fbtype NMTOKEN 'BUSY'
   ">


   <!-- Valid token values are "FREE", "BUSY", "BUSY-UNAVAILABLE", -->
   <!-- "BUSY-TENTATIVE", a non-standard "X-" name or another -->
   <!-- IANA registered name. -->

   <!ENTITY % attr.language "
   language CDATA #IMPLIED
   ">
   <!-- language value is a valid RFC 1766 language string -->

   <!ENTITY % attr.member "
   member CDATA #IMPLIED
   ">
   <!-- member value is one or more calendar user addresses -->

   <!ENTITY % attr.partstat "
   partstat NMTOKEN 'NEEDS-ACTION'
   ">
   <!-- Valid token value for VEVENT: "NEEDS-ACTION", "ACCEPTED", -->
   <!-- "DECLINED", "TENTATIVE", "DELEGATED", a non-standard "X- -->
   <!-- name or another IANA registered name. -->

   <!-- Valid token value for VTODO: "NEEDS-ACTION", "ACCEPTED", -->
   <!-- "DECLINED", "TENTATIVE", "DELEGATED", "COMPLETED", -->
   <!-- "IN-PROGRESS, a non-standard "X- name or another IANA -->
   <!-- registered name. -->
   <!-- Valid token value for VJOURNAL: "NEEDS-ACTION", "ACCEPTED", -->
   <!-- "DECLINED", a non-standard "X- name or another IANA -->
   <!-- registered name. -->

   <!ENTITY % attr.range "
   range NMTOKEN 'THISONLY'
   ">
   <!-- Valid token values are "THISONLY" or "THISANDPRIOR" or -->
   <!-- "THISANDFUTURE" -->

   <!ENTITY % attr.related "
   related NMTOKEN 'START'
   ">
   <!-- Valid token values are "START" or "END" -->

   <!ENTITY % attr.reltype "
   reltype NMTOKEN 'PARENT'
   ">
   <!-- Valid token values are "PARENT", "CHILD", SIBLING", -->
   <!-- a non-standard "X-" name or any IANA registered name. -->

   <!ENTITY % attr.role "
   role NMTOKEN 'REQ-PARTICIPANT'
   ">


   <!-- Valid token values are "CHAIR", "REQ-PARTICIPANT", -->
   <!-- "OPT-PARTICIPANT", "NON-PARTICIPANT", a non-standard "X-" -->
   <!-- name or any IANA registered name. -->

   <!ENTITY % attr.rsvp "
   rsvp NMTOKEN 'FALSE'
   ">
   <!-- Valid token values are "TRUE" or "FALSE", -->

   <!ENTITY % attr.sent-by "
   sent-by CDATA #IMPLIED
   ">
   <!-- sent-by value is a calendar user address -->

   <!ENTITY % attr.tzid "
   tzid CDATA #IMPLIED
   ">
   <!-- tzid value is a time zone identifier -->

   <!ENTITY % cal.comp "
   vevent | vtodo | vjournal | vfreebusy | vtimezone
   ">

   <!ENTITY % vevent.opt1 "
   class | created | description | dtstamp | dtstart | geo |
   last-modified | location | organizer | priority | recurrence-id |
   sequence | status | summary | transp | uid | url |
   (dtend | duration)
   ">
   <!-- These properties may only appear once in a VEVENT -->
   <!ENTITY % vevent.optm "
   attach | attendee | categories | comment | contact |
   exdate | exrule | rdate | related-to | resources | request-status |
   rrule
   ">
   <!-- These properties may appear one or more times in a VEVENT -->

   <!ENTITY % vtodo.opt1 "
   class | completed | created | description | dtstamp | dtstart |
   geo | last-modified | location | organizer | percent | priority |
   recurrence-id | sequence | status | summary | uid | url |
   (due | duration)
   ">
   <!-- These properties may only appear once in a VTODO -->

   <!ENTITY % vtodo.optm "
   attach | attendee | categories | comment | contact |
   exdate | exrule | request-status | related-to | resources |
   rdate | rrule


   ">
   <!-- These properties may appear one or more times in a VTODO -->

   <!ENTITY % vjournal.opt1 "
   class | created | description | dtstart | dtstamp | last-modified |
   organizer | recurrence-id | sequence | status | summary | uid | url
   ">
   <!-- These properties may only appear once in a VJOURNAL -->

   <!ENTITY % vjournal.optm "
   attach | attendee | categories | comment | contact |
   exdate | exrule | related-to | rdate | rrule | request-status
   ">
   <!-- These properties may appear one or more times in a VJOURNAL -->

   <!ENTITY % vfreebusy.opt1 "
   contact | dtstamp | dtstart | dtend | duration |
   organizer | uid | url
   ">
   <!-- These properties may only appear once in a VFREEBUSY -->

   <!ENTITY % vfreebusy.optm "
   attendee | comment | freebusy | request-status
   ">
   <!-- These properties may appear one or more times in a -->
   <!-- VFREEBUSY -->

   <!ENTITY % vtimezone.man "
   tzid
   ">
   <!-- These properties must appear in a VTIMEZONE -->

   <!ENTITY % vtimezone.opt1 "
   last-modified | tzurl
   ">
   <!-- These properties may only appear once in a VTIMEZONE -->

   <!ENTITY % vtimezone.mann "
   (standard | daylight), (standard | daylight)*
   ">
   <!-- These properties must appear in a VTIMEZONE and may -->
   <!-- appear multiple times -->

   <!ENTITY % standard.man "
   dtstart | tzoffsetto | tzoffsetfrom
   ">
   <!-- These properties must appear in a STANDARD, but only once -->

   <!ENTITY % standard.optm "


   comment | rdate | rrule | tzname
   ">
   <!-- These properties may appear one or more times in a STANDARD -->

   <!ENTITY % daylight.man "
   dtstart | tzoffsetto | tzoffsetfrom
   ">
   <!-- These properties must appear in a DAYLIGHT, but only once -->

   <!ENTITY % daylight.optm "
   comment | rdate | rrule | tzname
   ">
   <!-- These properties may appear one or more times in a DAYLIGHT -->

   <!ENTITY % audio.man "
   action, trigger
   ">
   <!-- These properties must appear in an audio VALARM. -->

   <!ENTITY % audio.optx "
   duration | repeat
   ">
   <!-- These properties may appear once in an audio VALARM. If one -->
   <!-- appears, then both must appear. -->

   <!ENTITY % audio.opt1 "
   attach
   ">
   <!-- These properties may appear once in an audio VALARM. -->

   <!ENTITY % valarm.audio "
   (%audio.man;), (%audio.optx;)*, (%audio.opt1;)
   ">

   <!ENTITY % display.man "
   action, description, trigger
   ">
   <!-- These properties must appear in a display VALARM. -->
   <!ENTITY % display.optx "
   duration | repeat
   ">
   <!-- These properties may appear once in a display VALARM. If -->
   <!-- one appears, then both must appear. -->

   <!ENTITY % valarm.display "
   (%display.man;), (%display.optx;)*
   ">

   <!ENTITY % email.man "
   action, description, summary, trigger
   ">
   <!-- These properties must appear in an email VALARM. -->

   <!ENTITY % email.optx "
   duration | repeat
   ">
   <!-- These properties may appear once in an email VALARM. If one -->
   <!-- appears, then both must appear. -->

   <!ENTITY % email.optm "
   attach
   ">
   <!-- These properties may appear one or more times in an email -->
   <!-- VALARM. -->

   <!ENTITY % email.mann "
   attendee
   ">
   <!-- These properties must appear in an email VALARM. The may -->
   <!-- appear more than once. -->

   <!ENTITY % valarm.email "
   (%email.man;), (%email.optx;)*, (%email.optm;)*,
   (%email.mann;)*
   ">

   <!ENTITY % procedure.man "
   action, attach, trigger
   ">
   <!-- These properties must appear in an audio VALARM. -->

   <!ENTITY % procedure.optx "
   duration | repeat
   ">
   <!-- These properties may appear once in an procedure VALARM. -->
   <!-- If one appears, then both must appear. -->

   <!ENTITY % procedure.opt1 "
   description
   ">
   <!-- These properties may appear once in a procedure VALARM -->
   <!ENTITY % valarm.procedure "
   (%procedure.man;), (%procedure.optx;)*, (%procedure.opt1;)?
   ">

   <!-- ******************************************** -->
   <!-- iCalendar value type notation declarations   -->
   <!-- ******************************************** -->


   <!-- NOTE: The "XCAL" text in the following NOTATION values
   will be replaced with the text "RFC xxxx", where "xxxx" is the RFC
   number, when this memo is published as a RFC. -->

   <!NOTATION BINARY PUBLIC "-//IETF//NOTATION XCAL/Value Type/Binary//EN">

   <!NOTATION BOOLEAN PUBLIC "-//IETF//NOTATION XCAL/Value Type/Boolean//EN">

   <!NOTATION CALADR PUBLIC "-//IETF//NOTATION XCAL/Value Type/Calendar
   User Address//EN">

   <!NOTATION DATE PUBLIC "-//IETF//NOTATION XCAL/Value Type/Date//EN">

   <!NOTATION DATE-TIME PUBLIC "-//IETF//NOTATION XCAL/Value
   Type/Date-Time//EN">

   <!NOTATION DURATION PUBLIC "-//IETF//NOTATION XCAL/Value
   Type/Duration//EN">

   <!NOTATION FLOAT PUBLIC "-//IETF//NOTATION XCAL/Value Type/Float//EN">

   <!NOTATION INTEGER PUBLIC "-//IETF//NOTATION XCAL/Value Type/Integer//EN">

   <!NOTATION PERIOD PUBLIC "-//IETF//NOTATION XCAL/Value
   Type/Period of Time//EN">

   <!NOTATION RECUR PUBLIC "-//IETF//NOTATION XCAL/Value
   Type/Recurrence Rule//EN">

   <!NOTATION TEXT PUBLIC "-//IETF//NOTATION XCAL/Value Type/Text//EN">

   <!NOTATION TIME PUBLIC "-//IETF//NOTATION XCAL/Value Type/Time//EN">

   <!NOTATION URI PUBLIC "-//IETF//NOTATION XCAL/Value Type/URI//EN">

   <!NOTATION UTC-OFFSET PUBLIC "-//IETF//NOTATION XCAL/Value
   Type/UTC-Offset//EN">

   <!NOTATION X-NAME PUBLIC "-//IETF//NOTATION XCAL/Value Type/X-Name//EN">

   <!-- ************************************************* -->
   <!-- iCalendar property element/attribute declarations -->
   <!-- ************************************************* -->

   <!ELEMENT br EMPTY>
   <!-- Signifies a new line in the TEXT value content information -->

   <!-- Description component properties element type declarations -->
   
   
      <!ELEMENT attach (extref | b64bin)>
      <!-- extref holds a reference to an external entity that -->
      <!-- has the attachment. b64bin holds the inline BASE64 encoded -->
      <!-- binary data for the attachment as defined in RFC 2045. -->
   
      <!ELEMENT extref EMPTY>
      <!ATTLIST extref
      uri ENTITY #REQUIRED>
   
      <!ELEMENT b64bin (#PCDATA)>
      <!ATTLIST b64bin
      %attr.fmttype;
      value NOTATION (BINARY) #IMPLIED>
   
      <!ELEMENT categories (item)*>
   
      <!ELEMENT item (#PCDATA)>
      <!ATTLIST item
      %attr.language;
      value NOTATION (TEXT) #IMPLIED>
   
      <!ELEMENT class (#PCDATA)>
      <!ATTLIST class
      %attr.language;
      value NOTATION (TEXT) #IMPLIED>
   
      <!ELEMENT comment (#PCDATA)*>
      <!ATTLIST comment
      %attr.language;
      %attr.altrep;
      value NOTATION (TEXT) #IMPLIED>
   
      <!ELEMENT description (#PCDATA)*>
      <!ATTLIST description
      %attr.language;
      %attr.altrep;
      value NOTATION (TEXT) #IMPLIED>
   
      <!ELEMENT geo (lat, lon)>
   
      <!ELEMENT lat (#PCDATA)>
      <!ATTLIST lat value NOTATION (FLOAT) #IMPLIED>
      <!-- A decimal degree float number to 6 decimal places -->
   
      <!ELEMENT lon (#PCDATA)>
      <!ATTLIST lon value NOTATION (FLOAT) #IMPLIED>
      <!-- A decimal degree float number to 6 decimal places -->
   
      <!ELEMENT location (#PCDATA)>
   
      <!ATTLIST location
      %attr.language;
      %attr.altrep;
      value NOTATION (TEXT) #IMPLIED>
   
      <!ELEMENT percent (#PCDATA)>
      <!ATTLIST percent
      value NOTATION (INTEGER) #IMPLIED>
   
      <!ELEMENT priority (#PCDATA)>
      <!ATTLIST priority
      value NOTATION (INTEGER) #IMPLIED>
   
      <!ELEMENT resources (#PCDATA)>
      <!ATTLIST resources
      %attr.language;
      %attr.altrep;
      value NOTATION (TEXT) #IMPLIED>
   
      <!ELEMENT status (#PCDATA)>
      <!ATTLIST status
      %attr.language;
      %attr.altrep;
      value NOTATION (TEXT) #IMPLIED>
      <!-- Text value must match the valid values for the particular -->
      <!-- calendar component. -->
   
      <!ELEMENT summary (#PCDATA)>
      <!ATTLIST summary
      %attr.language;
      %attr.altrep;
      value NOTATION (TEXT) #IMPLIED >
   
      <!-- Data and time component property element type declarations -->
   
      <!ELEMENT dtstart (#PCDATA)>
      <!ATTLIST dtstart
      %attr.tzid;
      value NOTATION (DATE-TIME | DATE) "DATE-TIME">
   
      <!ELEMENT dtend (#PCDATA)>
      <!ATTLIST dtend
      %attr.tzid;
      value NOTATION (DATE-TIME | DATE) "DATE-TIME">
   
      <!ELEMENT due (#PCDATA)>
      <!ATTLIST due
      %attr.tzid;
      value NOTATION (DATE-TIME | DATE) "DATE-TIME">
   
      <!ELEMENT completed (#PCDATA)>
      <!ATTLIST completed
      value NOTATION (DATE-TIME) #IMPLIED>
   
      <!ELEMENT duration (#PCDATA)>
      <!ATTLIST duration
      value NOTATION (DURATION) #IMPLIED>
   
      <!ELEMENT freebusy (#PCDATA)>
      <!ATTLIST freebusy
      %attr.fbtype;
      value NOTATION (PERIOD) #IMPLIED>
   
      <!ELEMENT transp (#PCDATA)>
      <!ATTLIST transp
      value NOTATION (TEXT) #IMPLIED>
      <!-- Text value must be one of the valid enumerations. -->
   
      <!-- Time zone component property element type declarations -->
   
      <!ELEMENT tzid (#PCDATA)>
      <!ATTLIST tzid
      value NOTATION (TEXT) #IMPLIED>
   
      <!ELEMENT tzname (#PCDATA)>
      <!ATTLIST tzname
      %attr.language;
      value NOTATION (TEXT) #IMPLIED>
   
      <!ELEMENT tzoffsetfrom (#PCDATA)>
      <!ATTLIST tzoffsetfrom
      value NOTATION (UTC-OFFSET) #IMPLIED>
   
      <!ELEMENT tzoffsetto (#PCDATA)>
      <!ATTLIST tzoffsetto
      value NOTATION (UTC-OFFSET) #IMPLIED>
   
      <!ELEMENT tzurl EMPTY>
      <!ATTLIST tzurl
      uri ENTITY #REQUIRED>
   
      <!-- Relationship component property element type declarations -->
   
      <!ELEMENT attendee (#PCDATA)>
      <!ATTLIST attendee
      %attr.language;
      %attr.cn;
      %attr.role;
      %attr.partstat;
      %attr.rsvp;
      %attr.cutype;
      %attr.member;
      %attr.delegated-to;
      %attr.delegated-from;
      %attr.sent-by;
      %attr.dir;
      value NOTATION (CALADR) #IMPLIED>
   
      <!ELEMENT contact (#PCDATA)*>
      <!ATTLIST contact
      %attr.language;
      %attr.altrep;
      value NOTATION (TEXT) #IMPLIED>
   
      <!ELEMENT organizer (#PCDATA)>
      <!ATTLIST organizer
      %attr.language;
      %attr.cn;
      %attr.sent-by;
      %attr.dir;
      value NOTATION (CALADR) #IMPLIED>
   
      <!ELEMENT recurrence-id (#PCDATA)>
      <!ATTLIST recurrence-id
      %attr.tzid;
      %attr.range;
      value NOTATION (DATE-TIME | DATE) "DATE-TIME">
   
      <!ELEMENT related-to (#PCDATA)>
      <!ATTLIST related-to
      %attr.reltype;
      value NOTATION (TEXT) #IMPLIED>
   
      <!ELEMENT url EMPTY>
      <!ATTLIST url
      uri ENTITY #REQUIRED>
   
      <!ELEMENT uid (#PCDATA)>
      <!ATTLIST uid
      value NOTATION (TEXT) #IMPLIED>
   
      <!-- Recurrence component property element type declarations -->
   
      <!ELEMENT exdate (#PCDATA)>
      <!ATTLIST exdate
      %attr.tzid;
      value NOTATION (DATE-TIME | DATE) "DATE-TIME">
   
      <!ELEMENT exrule (#PCDATA)>
      <!ATTLIST exrule
      value NOTATION (RECUR) #IMPLIED>
   
      <!ELEMENT rdate (#PCDATA)>
      <!ATTLIST rdate
      %attr.tzid;
      value NOTATION (DATE-TIME | DATE) "DATE-TIME">
   
      <!ELEMENT rrule (#PCDATA)>
      <!ATTLIST rrule
      value NOTATION (RECUR) #IMPLIED>
   
      <!-- Alarm component property element type declarations -->
   
      <!ELEMENT action (#PCDATA)>
      <!ATTLIST action
      value NOTATION (TEXT) #IMPLIED>
      <!-- Text value must be a valid enumeration -->
   
      <!ELEMENT repeat (#PCDATA)>
      <!ATTLIST repeat
      value NOTATION (INTEGER) #IMPLIED>
   
      <!ELEMENT trigger (#PCDATA)>
      <!ATTLIST trigger
      %attr.related-to;
      value NOTATION (DURATION | DATE-TIME) "DURATION">
   
      <!-- Change management component property element type -->
      <!-- declarations -->
   
      <!ELEMENT created (#PCDATA)>
      <!ATTLIST created
      value NOTATION (DATE-TIME) #IMPLIED>
   
      <!ELEMENT dtstamp (#PCDATA)>
      <!ATTLIST dtstamp
      value NOTATION (DATE-TIME) #IMPLIED>
   
      <!ELEMENT last-modified (#PCDATA)>
      <!ATTLIST last-modified
      value NOTATION (DATE-TIME) #IMPLIED>
   
      <!ELEMENT sequence (#PCDATA)>
      <!ATTLIST sequence
      value NOTATION (INTEGER) #IMPLIED>
   
      <!-- Miscellaneous component property element type declarations -->
   
 
   
      <!ELEMENT request-status (#PCDATA)>
      <!ATTLIST request-status
      %attr.language;
      value NOTATION (TEXT) #IMPLIED>
   
      <!-- iCalendar object element type declarations -->
   
      <!ELEMENT iCalendar (vcalendar+)>
   
      <!ELEMENT vcalendar (%cal.comp;)*>
      <!ATTLIST vcalendar
      %attr.language;
      xmlns CDATA #FIXED 'http://www.ietf.org/internet-drafts/draft-ietf-calsch-many-xcal-00.txt'
      calscale CDATA "GREGORIAN"
      method CDATA "PUBLISH"
      version CDATA #REQUIRED
      prodid CDATA #IMPLIED>
      <!-- version - Must be "2.0" if document conforms to this spec. -->
      <!-- calscale - Calendar scale. Default is GREGORIAN. -->
      <!-- method - C&S method. Default is iTIP PUBLISH. -->
      <!-- prodid - ISO 9070 FPI for product that generated iCalendar. -->
   
      <!-- "vevent" element type declaration -->
      <!ELEMENT vevent ((%vevent.opt1;)*, (%vevent.optm;)*, valarm*)>
   
      <!-- "vtodo" element type declaration -->
      <!ELEMENT vtodo ((%vtodo.opt1;)*, (%vtodo.optm;)*, valarm*)>
   
      <!-- "vjournal" element type declaration -->
      <!ELEMENT vjournal ((%vjournal.opt1;)*, (%vjournal.optm;)*)>
   
      <!-- "vfreebusy" element type declaration -->
      <!ELEMENT vfreebusy ((%vfreebusy.opt1;)*, (%vfreebusy.optm;)*)>
   
      <!-- "vtimezone" element type declaration -->
      <!ELEMENT vtimezone (%vtimezone.man;, (%vtimezone.opt1;)*,
      (%vtimezone.mann;)*)>
   
      <!ELEMENT standard (((%standard.man;)*), (%standard.optm;)*)>
   
      <!ELEMENT daylight (((%daylight.man;)*), (%daylight.optm;)*)>
   
      <!ELEMENT valarm ((%valarm.audio;) | (%valarm.display;) |
      (%valarm.email;) | (%valarm.procedure;))>