It used to be a day that retailers were closed. But in the past few years U.S. retailers have been open at least for a few hours on Thanksgiving. Usually something like 6 pm to Midnight this year.
Best Buy here sold more items in that 6 hours on Thanksgiving that they did in the entire day on Black Friday.
Now Best Buy did cut back this year. Last year on Thanksgiving they opened at 6 pm and stayed open until 11 pm Black Friday night. But they did almost no business from Midnight to 8 am, thus the change this year. Costco enters into that picture because they refused to open on Thanksgiving, even though every one of their national store competitors did.