iCalendar

RFC 2447: iCalendar Message-Based Interoperability Protocol (iMIP)
Specifies a binding from the iCalendar Transport-independent Interoperability Protocol (iTIP) to internet email-based transports.
http://tools.ietf.org/html/rfc2447

RFC 2446: iCalendar Transport-Independent Interoperability Protocol (iTIP)
Specifies how calendaring systems can use iCalendar objects to interoperate with other calendar systems.
http://tools.ietf.org/html/rfc2446

RFC 2445: Internet Calendaring and Scheduling Core Object Specification (iCalendar)
Official specification for the format, including full syntax and recommendations.
http://tools.ietf.org/html/rfc2445

Wikipedia: iCalendar
Encyclopdia article on the format, with an overview of the specifications, and details of various implementations.
http://en.wikipedia.org/wiki/ICalendar