Matress sales are nothing more than a gimick to get you excited. You'll see matress sales advertised every single day in the newspaper (or at least every Sunday). And if you go into a matress store, you'll probably see that every single matress in the store is on sale ... every time you go there.
The list price of a matress is fictional number that they can reduce to make it "on sale".
Here's the best way to get a matress that's really on sale:
Remark that you didn't realize how expensive a matress would be. Say you're not sure if you want to pay quite that much right now. Then say you'll have to think about it and leave. Before you leave, you will most likely be offered a better deal.