Always allow for 53 weeks. It happens at least every ?5? years. Think:
Is 365.4 divisible by 7? No, it's 52.2. So (5 years) * (.2 weeks/year) =
1 extra week every 5 years. We used to do 13 four week period years and
every 5 years we had one 5 week period.
And, depending on which function you use, there can be 54 weeks!