LGA 1972 Schedule 12 Part 2 Paragraph 7 states:-
(1) A parish council shall in every year hold an annual meeting.
(2) In a year which is a year of ordinary elections of parish councillors, the annual meeting of a parish council shall be held on, or within fourteen days after, the day on which the councillors elected at that election take office, and in any other year the annual meeting shall be held on such day in May as the parish council may determine.
So it's a requirement of the legislation, but like so many issues in local council legislation, there isn't an obvious penalty for non-compliance.