re:" each quarter of the year has a period of 13 weeks, the first 2 months have 4 weeks and the last one has a 5 week period."
In that case I need to know how you number your weeks each year - I can't match it up with any of the standard week numbering conventions. I need this so that I'm sure it'll work properly in future years (I hope to calculate week numbers with formula to determine start dates and do away with the need for the intermediate pivot tables).