iCalendar spec: 4.8.5.4 Recurrence Rule
by far the best write-up (with lots of examples) on how the iCalendar recurrence format is set up.
iCalcreator
An open source iCal creator. Coincidentally, ICal has my vote for one of the worst specs in the history of specs. Here is the RFC that spells it all out. Here is another reference I liked. And of course, Wikipedia.