The day after Thanksgiving...busiest shopping day of the year in the US. It's called Black Friday because it's historically it's been the time when retailers begin to turn a profit for the year.
Stores have all sorts of huge sales (often with insanely-priced doorbuster deals on a few items to get people in the door) and open very early (many stores by 6 AM).
Thanks! Presumably it will only be the US Apple stores then.