I'm glad that my four-fours advent calendar enjoys a small following. I have even heard that certain programmers have been seen arriving at work before lunchtime, going straight to the calendar on the way in.
The original idea for the calendar came from the column Numbers Count in PCW magazine sometime in the eighties. A stylish column in a fine magazine: It published mathematical problems and occasional puzzles, and did not waste much space on answers. One of the puzzles was to count from 1 to 20 using four sevens. As I recall, I found 19 to be the most difficult.
I first tried to extend that to 24 to make an advent calendar, but had to give up on 23, which blocked me for a few years. Four fours was much easier. This year I tried four threes. Again, 23 looks poor, so but one can always choose to pay less attention to that and more to 24, for which I found several good possibilities. Which is the most elegant 24, 3!×(3⁄3+3) or 33-√3×3? Or 3√3×3-3 perhaps?