Real life holidays that are modified and changed to fit into game lore and aesthetics are my personal favorite. A Winter's Fest instead of Christmas or Love/Heart's Day instead of Valentine Day, etc., where it fits into the lore of the game and draws upon the game's religions and gods and traditions and has it's own unique take on the aesthetic!
My favorite part of the Holidays in December is logging into Overwatch to the sound of holiday music and new holiday skins, while all my friends log on too since they have time off. Such a nice feeling!