RRule for semi-weekly events

Question

I've been trying to figure out how to compose an RRULE that has events on alternating days each week; for example:

• Week A: Monday at 4pm
• Week B: Friday at 4pm

Where Week A and Week B alternate continuously into the future.

The closest I've gotten is this:

RRULE:FREQ=WEEKLY;COUNT=30;INTERVAL=2;BYDAY=MO,FR

which is close, but gives me the monday and friday on the same week. How can I code it to select the second friday in the two-week interval? This seems much less complex than much of what can be done with RRULEs, so I'm likely missing something obvious?

Answer 1

If this is intended to go on forever, with no 'COUNT', then I think the only way you can do it is to have two events.

Using any other of the BY's even BYSETPOS gets one into trouble at some point at the end of the year, or the end of the sequence as far as you defined it. Two events allows one to iterate the Monday and the Friday cleanly.

The choice of DTSTART for each event is crucial to be sure that you are recurring the right Fridays after the Mondays. You have something like M fm FM fm FM fm

From https://calendar.google.com/calendar/ical/eppua87dpfnq85bsrrt2siqlcs%40group.calendar.google.com/public/basic.ics

BEGIN:VEVENT
DTSTART:20211022T160000Z
DTEND:20211022T170000Z
RRULE:FREQ=WEEKLY;WKST=MO;INTERVAL=2;BYDAY=FR
DTSTAMP:20211012T064353Z
UID:STACKTESTFRIDAY
CREATED:20171102T185254Z
DESCRIPTION:
LAST-MODIFIED:20211012T064141Z
LOCATION:
SEQUENCE:2
STATUS:CONFIRMED
SUMMARY:Afternoon Tea on Friday
TRANSP:OPAQUE
END:VEVENT
BEGIN:VEVENT
DTSTART:20211011T160000Z
DTEND:20211011T180000Z
RRULE:FREQ=WEEKLY;WKST=MO;INTERVAL=2;BYDAY=MO
DTSTAMP:20211012T064353Z
UID:STACKTESTMONDAY
CREATED:20171102T185254Z
DESCRIPTION:
LAST-MODIFIED:20211012T064112Z
LOCATION:
SEQUENCE:2
STATUS:CONFIRMED
SUMMARY:Afternoon Tea on Monday
TRANSP:OPAQUE
END:VEVENT
END:VCALENDAR

Answer 2

Here another option that might work for you. In a YEARLY rule you can prepend the BYDAY elements with an offset n to identify the n-th instance of that weekday in the year.

If your recurring event starts on the 20th Monday of the year you could build a rule like this to iterate 30 instances with alternating weekdays.

RRULE:FREQ=YEARLY;BYDAY=20MO,21FR,22MO,23FR,24MO,25FR,26MO,27FR,28MO,29FR,30MO,31FR,32MO,33FR,34MO,35FR,36MO,37FR,38MO,39FR,40MO,41FR,42MO,43FR,44MO,45FR,46MO,47FR,48MO,49FR

Note, this assumes the year starts on a Saturday, Sunday or Monday.

If the year starts on a Tuesday, Wednesday, Thursday or Friday, the weekdays have to have the same number (because in that case the n-th Monday and the n+1-th Friday occur in the same week), i.e.

RRULE:FREQ=YEARLY;BYDAY=20MO,22FR,22MO,24FR,24MO,26FR,26MO,28FR,28MO,30FR,30MO,32FR,32MO,34FR,34MO,36FR,36MO,38FR,38MO,40FR,40MO,42FR,42MO,44FR,44MO,46FR,46MO,48FR,48MO,50FR