Currently when creating a recurring event in the Calendar and Events plugin it's not possible to have an event that repeats on (for example) the first Tuesday of every month. This is a standard recurrence pattern in most common scheduling and calendar apps so it seems like a strange omission from Discourse.
This is a feature that has been requested in numerous other topics and posts, so there's apparently demand for it. For instancehttps://meta.discourse.org/t/can-recurring-events-created-by-first-second-third-fourth-m-t-w-th-f-of-every-month/292887https://meta.discourse.org/t/discourse-calendar-and-event/97376/486?u=stephen34https://meta.discourse.org/t/recurring-events-not-displaying-in-the-calendar/343255/2?u=stephen34
Discuss this on our forum.