The thing is, there's a much better solution to this, that solves the user choice problem easily and fixes every conceivable problem that the date-based setup caused. Even better, it's a feature the entire community already knows and is quite familiar with: a third-party WAD file. You can use glaber's aforementioned (and awful) mod, or if you want you can make your own that isn't as sucky. Either way you get everything that we mentioned before with none of the complicated problems that the built-in, system clock setup created.
^This. Anybody can make a WAD that can replace the graphics of the official levels to be snow/Christmas-themed. The best part is the fact that it's not triggered via the system clock; it can be added at any time of the year. In fact, I suggest that if somebody were to make a Christmas mode WAD, make the Christmas mode maps be on a separate rotation from the rest of the game and maybe make it an unlockable.
I realize this opens an entirely different can of worms, but more than a Christmas-themed level or level-recolor, I'd like one "official" single-player snow-themed level, even if it were short and/or unlockable. I would think that there'd be enough inherent platforming material in such a level (slippery surfaces, traversing under ice floes, maybe an SRB2 Riders-esque boarding section, etc, etc) that one wouldn't be forced to simply make a Midnight Freeze-clone. I honestly thought the original idea with Blue Mountain Zone was to combine lava and ice elements and I loved it.
IIRC, there was a snow/Christmas-themed level that was an unlockable in 1.09.4 days, the Christmas Hunt Zone. In that level you had to hunt down three emeralds that were scattered throughout the map a la Knuckles's stages in Sonic Adventure and Sonic Adventure 2.