RRULEFREQMONTHLY

RRULEFREQMONTHLY refers to the monthly recurrence frequency used in the iCalendar RRULE component, as defined by RFC 5545. It specifies that an event repeats on a monthly basis, with optional modifiers to refine which month’s occurrence and which day within the month are chosen.

In an RRULE line, FREQ=MONTHLY sets the recurrence to monthly. The INTERVAL parameter can shift the cadence to every N months (default is 1). End conditions are provided by UNTIL (an absolute end date) or COUNT (a total number of occurrences). The monthly pattern can be refined with several parts: BYMONTHDAY, BYDAY, BYMONTH, and BYSETPOS. BYMONTHDAY selects a specific day number within the month, and negative values count from the end of the month (for example, -1 is the last day). BYDAY allows specifying days of the week with an ordinal, enabling patterns such as the second Tuesday of each month (BYDAY=2TU) or the first Monday (BYDAY=1MO). When BYSETPOS is used, it selects among the occurrences that result from the other rules, enabling constructs like “the first Monday of each month” by combining BYDAY with BYSETPOS.

Examples include: FREQ=MONTHLY; INTERVAL=1; BYMONTHDAY=15 (the 15th of every month); FREQ=MONTHLY; BYDAY=2MO (the second Monday of every month); FREQ=MONTHLY; BYDAY=MO; BYSETPOS=1 (the first Monday of every month); FREQ=MONTHLY; BYMONTHDAY=-1 (the last day of each month).

Compatibility notes: most calendar applications and libraries support FREQ=MONTHLY, but interpretation of ordinal BYDAY values and BYSETPOS can vary slightly between implementations. It is advisable to test complex patterns across platforms.