BYMONTHBYDAY

BYMONTHBYDAY is a component of the iCalendar specification's recurrence rule (RRULE) used to specify days of a month by their ordinal weekday. It allows scheduling events on the nth occurrence of a particular weekday within each month, rather than by an absolute day number.

Syntax and usage: Each entry in BYMONTHBYDAY consists of an ordinal followed by a weekday, such as 1MO, 2WE, -1FR. The ordinal can be positive (the nth occurrence) or negative (counting from the end of the month, e.g., -1FR for the last Friday). When combined with a monthly or yearly frequency, BYMONTHBYDAY selects the corresponding weekdays within each month that the rule applies to.

Examples: FREQ=MONTHLY; BYMONTHBYDAY=1MO selects the first Monday of every month. FREQ=MONTHLY; BYMONTHBYDAY=-1FR selects the last Friday of every month. More complex rules can include multiple entries, generating multiple occurrences within a single month (e.g., 1MO, -1FR).

Relationship to other RRULE parts: BYMONTHBYDAY is one of several BY* components in RRULE. It is commonly used for “the nth weekday of the month” patterns and can be combined with BYMONTH to target specific months or with BYDAY and BYSETPOS depending on implementation. Some calendar systems implement this directly, while others simulate the effect with BYDAY and BYSETPOS.

Compatibility and notes: BYMONTHBYDAY is defined in the iCalendar specification but support may vary across calendar clients and libraries. Where BYMONTHBYDAY is not supported, similar schedules can often be expressed using BYDAY in combination with BYSETPOS.

See also: iCalendar RRULE, BYDAY, BYMONTH, BYMONTHDAY, BYSETPOS.