I get it, but Advent calendars are not just German. I am pretty sure they are common all over the western world where Xmas is celebrated. But Advent doesn't in any way mean 25.

They're definitely not German now. They may have been originally, but these days, they're first and foremost a commercial phenomenon, meaning they went global. Chocolate advent calendars. Trinket advent calendars. DIY advent calendars. ${insert your most hated kids toys franchise} advent calendars. And so on.