Banjer: I haven't read the site yet, but "Every fifth Sunday of the month" is quite a common system. (also every fifth Monday, Tuesday etc.,)

Many premises (pubs, clubs) have regular bookings for once or twice a month, which would normally be "First Tuesday of every month" or "First & Third Tuesday of every month". If the premises can get a complementary booking "Second and Fourth Tuesday of every month" then four or five times a year they get an unbooked Tuesday on the fifth Tuesday. Occasional bookings can fill these gaps, which are not evenly spaced throughout the year.

CHEERS

Nigel