To be fair in the original Advent of Code the last day is just a freebie and not a real programming problem.

Right, mini-spoiler if you've never reached 50 stars for a year:

The 50th star is awarded for having the other 49 stars and then basically clicking OK I think. If you've been diligently solving them in order, it means you effectively get two stars for your final 49th puzzle of the year on Christmas Day, which makes sense because then the puzzles are very hard and a "normal" puzzle wouldn't leave much time for other Christmas Day activities. But if you're the sort of person who often gives up on a day and never comes back you may never have seen this because you never got to 49 stars.

You're not quite right that the Christmas Day puzzle is trivial - it's the first half of a maybe week 1 type difficulty puzzle, but there just isn't a second half:

Here's the puzzle text for last year's Xmas Day (if you are logged in you can play, but even without it will explain the puzzle it just won't give you an input to test your solution):

https://adventofcode.com/2024/day/25

Right so since each problem has more than one part, we are back to advent of 24 problems.

Really though, for Catholic and protestant churches, Advent starts the fourth Sunday before Christmas, so isn't always the same length. In Orthodox Churches, Advent is 40 days (starting Nov 15) just like Lent.