Not in all traditions. E.g. in Germany Christmas Eve is typically the last day so you only get 24.
Yeah, in Sweden too.
Norway, too.
And even in the places where it’s 25 days, there’s plenty of advent calendars that only have 12 doors - though they’re typically budget versions of expensive calendars (eg a dram of whisky behind each door)
Falsehoods programmers believe about time #604: all advent calendars have 25 slots
Winter Solstice is usually (not always) on 21th December.