RRULE

RRULE stands for Recurrence Rule and is a fundamental part of the iCalendar data format used to describe repeating events. It is expressed as a string assigned to the RRULE property of a VEVENT or VTODO component and consists of key=value pairs separated by semicolons. The FREQ key is required and defines the frequency of repetition, with values such as SECONDLY, MINUTELY, HOURLY, DAILY, WEEKLY, MONTHLY, or YEARLY. Additional keys control the end of the recurrence (UNTIL or COUNT), the spacing between occurrences (INTERVAL), and how dates are filtered within each period (BYDAY, BYMONTH, BYMONTHDAY, BYWEEKNO, BYYEARDAY, BYHOUR, BYMINUTE, BYSECOND, BYSETPOS, and related variants).

UNTIL specifies an end date/time for the series, while COUNT sets a maximum number of occurrences. INTERVAL indicates the gap between repetitions; for example FREQ=WEEKLY;INTERVAL=2 means every other week. BYxxx parts refine the pattern, such as BYDAY to pick specific weekdays (e.g., FREQ=WEEKLY;BYDAY=MO,WE,FR for Mondays, Wednesdays, and Fridays) or BYMONTHDAY to select certain days of the month. Complex rules can combine multiple BY components or use BYSETPOS to select specific occurrences within a period.

RDATE can be used alongside RRULE to include additional dates outside the primary rule. RRULEs are evaluated from the event’s DTSTART, and the resulting instances typically respect the event’s time zone. While broadly supported, implementations may vary in handling certain BY rules and edge cases.

Understanding RRULEs enables precise sharing and synchronization of repeating events across calendar applications.