RFCs describe all formats in a formal way and also often includes examples, but when it comes to the question if e.g. a ics file found in the wild is valid or not, external sources like validators are very useful for debugging.
Here you will find a collection of the ones, we use and think are useful to you too:
Based on the current iCal4J version by Ben Fortuna
Note: this service is unavailable as of 2018-09-27
Z Content (Dan Cogliano) is a developer of iCalendar tools for the Joomla Content Management System.