Epic Games Store free games refresh every Thursday at 8 AM PT / 11 AM ET / 4 PM GMT / 5 PM CET. Each free game is available for exactly one week before being replaced. If you miss the claim window the game returns to its regular price. During the Christmas holiday period (usually December 11–31), Epic gives away a free game every single day instead of weekly. Once you claim a game during its free period it stays in your library permanently regardless of when you download it.
