Ics Specification RFC 5545

• RFC 5545 Ical Specifications (replaced 2445)
• kanzaki.com’s easier to read selected RFC5545
• Wikipedia overview (fairly handy)
• CalConnectSM, The Calendaring and Scheduling Consortium
• iCal Clients Recurrence Rules Interoperability Matrix
• Ical Requirements (the shoulds, musts, and must not)

Ics (Icalendar) Validators:

These are helpful but not necessarily complete.  They have been described by some as ‘trailing edge’.  At the end of the day you need to ensure that your ics file will be accepted by the major calendar applications (eg: google calendar etc)

• ICal Validator
• icalendar.org validator
• herokuapp.com validator

Other related ics resources

• Microformats at schema.org (may help with google event seo)
• HCalendar microformat
• Vcalendar
• Googles notes on the search results event data highlighter
• Rich snippets for events  (here). Works well, sometimes too well. If you are a low traffic site – google may be your biggest visitor!
• Timezones in .ics files (here)

Development and Testing Resources

• iCalendar Recurrence Rule Tests
• Payday Example
• Tests for Recurrence Updates and Exceptions
• textmagic.com’s rrule generator

How to subscribe to calendars

• using google
• using an apple mac
• using outlook

Useful ics files for testing

• https://support.google.com/calendar/answer/37098?hl=en
• http://www.calendarlabs.com/ical-calendar-holidays.php

Other discussions

• https://icalevents.com/0d0a-in-your-content-andencodingquoted-printable/
• http://icalvalid.wikidot.com/