RRULEFREQMONTHLYBYDAY1MO1FR

RRULEFREQMONTHLY, typically written as FREQ=MONTHLY in an RRULE, denotes a monthly recurrence pattern used in iCalendar and related calendaring systems. An RRULE is paired with a start date (DTSTART) to generate future event instances. The FREQ value determines the base cadence; MONTHLY means events repeat roughly once per calendar month, with optional modifiers to refine the pattern.

Key modifiers for monthly rules include INTERVAL, BYMONTHDAY, BYDAY, BYMONTH, and BYSETPOS. INTERVAL specifies how many months between occurrences (default is 1). BYMONTHDAY selects specific day(s) of the month, for example BYMONTHDAY=15 schedules the event on the 15th of every month. BYDAY selects day(s) of the week, optionally with an ordinal, to express patterns such as the second Monday or the last Friday. BYSETPOS can narrow a generated set of dates to a particular position, enabling common patterns like the second Monday of each month when used with BYDAY. BYMONTH restricts occurrences to certain months if needed.

Common example patterns include: FREQ=MONTHLY; INTERVAL=1; BYMONTHDAY=1 for the first day of every month, and FREQ=MONTHLY; BYDAY=MO; BYSETPOS=1 for the first Monday of each month, or FREQ=MONTHLY; BYDAY=FR; BYSETPOS=-1 for the last Friday of each month. Complex patterns may combine BYDAY with BYSETPOS to express ordinal weekdays within the month.

A start date (DTSTART) anchors the rule, and UNTIL or COUNT can bound the recurrence. Compatibility varies by library, but FREQ=MONTHLY is a standard component of the iCalendar RRULE specification (RFC 5545).