I think the easiest way to have 24/25 of something would be to have the "part 2" for each puzzle get released the next day. It would probably ruin the momentum about as much as having days off would (as another alternative to make the timing fit "advent"), but there could be a fun extra layer of puzzle with a hint of what the part 2 will be so people can try to speculate and modify their code in anticipation.
I am not aware of Eric saying something about that alternative, but this comment on reddit[1] makes a lot of sense to me:
> Given that part 2 is often a very simple modification of part 1, this could lead to many of the days being total letdowns. I can enjoy a simple puzzle, but I'd be a bit disappointed if one day is a single line change to the previous day.
I'd also add that not having to be worried everyday about something makes a lot of sense. He can have fewer days "on call" in December with.
