It's not just marked-down junk. I lived for 40 years in the States and a lot of people do all their Christmas shopping on the day after (U.S.) Thankgiving. It's sort of like the American version of Boxing Day.
I've gotten some emails from Canadian stores (e.g. Mostly Digital in the Toronto area) offering discounts tomorrow as well, so it seems to be catching on in parts of Canada. Apple Canada also has a sale tomorrow.
The issue of course is, if you just drive down and back in the same day, you can't bring back very much with you without having to pay duty. If you're going to buy a lot of stuff, you should spend the whole weekend, and unless you're staying with friends for free, you'll end up spending a lot more on hotel and food than you'll save during the sales.
Plus you'll have to declare your purchases at the border and pay Canadian sales taxes on them unless their under the limit. Plenty of people go through the border and lie, saying they didn't buy anything, but I imagine the agents will be a bit more skeptical of such claims this weekend. We've had our car searched a few times coming back into Canada, and one agent even looked at the tags of the clothes we were wearing to try to determine whether we'd bought them in the States and were wearing them to elude taxes at the border. So I declare everything and don't take any chances...it's not worth the fines and hassle if they catch you.