iCalendar (RFC 5545) specifies a standard method for representing and exchanging calendaring and scheduling information. This RFC supercedes the original iCalendar specification , RFC 2445. Original text version, iCalendar.org's HTML version
New Properties for iCalendar (RFC 7986) defines a set of new properties for iCalendar data and extends the use of some existing properties to the entire iCalendar object. Original text version, iCalendar.org's HTML version
CalDAV (RFC 4791) defines extensions to the Web Distributed Authoring and Versioning (WebDAV) protocol for use with calendar and scheduling information. Original text version, iCalendar.org's HTML version
Validation tools help developers find errors in their feeds and to ensure compliance with the iCalendar standard.
http://icalendar.org/validator.html - an iCalendar validator written by Dan Cogliano.
Programming libraries can help reduce development time building applications featuring iCalendar support. This list is informational only and does not imply any recommendation on these libraries.
C/C++ - Libical http://libical.github.io/libical/
Java - iCal4j https://github.com/ical4j/ical4j/wiki
.NET - iCal.NET https://github.com/rianjs/ical.net
Python - iCalendar package https://pypi.python.org/pypi/icalendar
Ruby - icalendar http://icalendar.rubyforge.org/
National Public Holidays - An extensive list of national and state public holidays in icalendar format.
WebCal.Guru- A popular site listing calendars for holidays, science, sports and more.
iCalShare.com - A popular site listing thousands of calendars
CalConnect - A calendaring and scheduling consortium
Internet Engineering Task Force (IETF) - The organization responsible for publishing the iCalendar RFC and other standard protocols
Calendar Swamp - A blog about calendar issues and interoperability calendarswamp.blogspot.com