Here is what I've seen happen, the stores gets a projected shipment schedule from the manfacturer, so the manufacturer makes their ads a few months in advance. If they were expecting to have quite a few of something on a specific sales date, ten the might have to hold on to a extra shipment just to make sure they have the "sale" item available on th "sale" day. Thats why some stores my get a shipment in on a Wed. but have to lie and say that they don't have any in, cause they aren't allowed to sell them until that Sunday. If they are lucky they also get another shipment in on that Sunday and now they can have a really big "sale" day for the shopper.
This is why BB hordes their systems, so that they have the largest possible inventory for the sales day. or what some might refer to as their "weekly re-launch". It also does't help the hype that people are constantly lining up outside of the stores on a weekly basis trying to secure a wii, and will proably continue to do so up until the middle of January. Has there ever been anything like this for any other system ever?