I don't think there is a blanket answer but here is what I have found over my years (13!) of shopping.
February, April and August are consisently my best months. The first two weeks of my month are usually my busiest with the third being pretty slow. That is the week I plan vacations if I am going somwhere I don't really care to incorporate shopping. The fourth can be great with last minute shops and bonuses. End of quarters can also be really good bonus wise.
December and May can be great months if you are willing to shop...seems like the holidays and weather of December and the dance recitals, graduations, end of school events of May keep a lot of regular shoppers busy that month. December bonuses often go up fast and early in the month but those few days around Christmas can bring some huge suprise bonuses out of nowhere...think shops you would have thought done and over but there must have been a problem and there is now a reshop with big bucks.
January usually turns out ok but it starts with everyone (well a lot of people) on the boards complaining that they can't find any shops, the companies must be changing, etc and so forth. It is true that some companies switch at the first of the year but honestly I have seen it happen all throughout the year so January doesn't bother me.
The other constant I have noticed over the years is every time gas prices skyrocket there are threads about people no longer shopping yet it seems the industry does move forward despite the gas prices.
Liz