RRULEs

RRULE is the recurrence rule property used in iCalendar (RFC 5545) to describe how an event repeats. It encodes a set of constraints as a string of key=value pairs, to be used with the event’s start time (DTSTART) to generate all occurrences.

A typical RRULE begins with FREQ, which must be one of DAILY, WEEKLY, MONTHLY, or YEARLY. Additional parameters control interval, end condition, and refinement: INTERVAL sets how many units between occurrences (default 1); UNTIL specifies the last date-time in the series; COUNT specifies the total number of occurrences; BYDAY, BYMONTH, BYMONTHDAY, BYHOUR, BYMINUTE, and BYSECOND refine the pattern (for example MO, WE, FR for a weekly schedule; 1MO for the first Monday).

Common examples illustrate its use. FREQ=WEEKLY;BYDAY=MO,WE,FR means the event recurs every week on Monday, Wednesday, and Friday. FREQ=MONTHLY;BYDAY=1MO means the first Monday of each month. FREQ=DAILY;INTERVAL=2;COUNT=10 means the event occurs every two days for ten occurrences.

End conditions and exceptions are supported through UNTIL and COUNT, and calendars may include RDATE for additional dates and EXDATE for exclusions. RRULEs are interpreted by calendar applications to generate the actual sequence of event instances, taking into account time zones and the DTSTART value. They are a core mechanism for expressing recurring events in shared calendars and scheduling systems.