I probably buy most stuff on a weekend, but it has nothing to do with when I get paid.  (That's what credit cards are for. ;-)  The weekend is when I have time to go through the various stores and see what's on offer. 

I do the bulk of my Poser purchases during the holiday season.  That's when the best sales are.  Content Paradise usually has a 50% off sale, and so does RDNA.  DAZ runs a lot of sales in Nov. and Dec.  Rosity usually offers some nice coupons and discounts.  If it's something I like but don't need right away, I wait until the holiday season to see if it will go on sale.

I put aside some money monthly, all year long, waiting for the holiday sales period.