If you visit the Epic Games Store, go to the section for free games. You’ll find something aside from the regular free games.
The free game for the next week is not shown. Instead, you have an image with Christmas decorations like last year. It’s called “Mystery Game”.
If Epic Games repeats last year’s approach, the next free game will only be available for 24 hours. After this period, a new free game will become available.
If that happens, you will have to be fast, or you’ll be missing out on probably AAA games.