These festivals are often celebrated in larger communities, all the covens of a geographical area getting together to celebrate. The covens celebrate the moon cycles. Each coven picks it's celebration time according to how they wish to work. There are Waxing Moon covens, Full moon covens, Waning moon covens and Dark Moon covens. Often they are very close friends.
Covens have a very variable cycle.
Some gather once for a specific reason and never again. Others stay viable for many years and even go into generations. There are some very long-lived covens in the California Bay Area and the Massachusetts area. A Circle is a gathering of members of different covens and people who are solitairy. Circles gather on a regular schedule, but don't have a regular attendance.