BYMONTHDAY25

BYMONTHDAY25 is a reference to a value in the iCalendar RRULE (recurrence rule) syntax used to refine monthly occurrences by specifying which days of the month to include. In this context, BYMONTHDAY indicates the day numbers within a month that are eligible for an event to occur. A single value such as 25 means the event is scheduled on the 25th day of each relevant month. BYMONTHDAY also accepts multiple values (for example, BYMONTHDAY=1,15 would allow occurrences on the 1st and the 15th days of each month). Negative values are allowed to count from the end of the month (for instance, -1 represents the last day of the month, -2 the second-to-last day, etc.).

When used with a recurrence frequency (FREQ), BYMONTHDAY constrains the dates generated by the rule. For example, FREQ=MONTHLY; BYMONTHDAY=25 yields events on the 25th day of every month. If a month does not have a specified day (such as 31 in months with only 30 days), that particular occurrence is not produced. BYMONTHDAY can be combined with other rule parts, such as BYMONTH and BYDAY, to create more complex schedules (for instance, selecting the 25th day in only certain months).

BYMONTHDAY is part of the iCalendar standard and is implemented in many calendar applications and scheduling libraries. It is commonly used for regular monthly reminders, billing cycles, or any schedule anchored to specific days within calendar months.