Mh... Roses has his building-subtype-change scripts, which can change workshops based on a timer. One could do "holiday decorations" that automatically change with time. Show a pumpkin lantern and spiderwebs for halloween, show pretty lights for christmas, etc. Or whatever holidays you will come up with.
There are of course seasonal plants for farmplots, which could be used to grow christmas trees etc in winter. SPRING, SUMMER, AUTUMN, WINTER also works on wild plants.
The same counts for grasses. For example in my Warlock fort I had a nice view of the mountain avens starting to bloom in winter, slowly spreading over the map.
Color changes on leaves on trees are a thing, just like their grows. You can easily make a conifer tree that "grows" pretty glass spheres and candles at a specific time of year, using GROWTH_PRINT and GROWTH_TIMING.
There are caste-specific creature tokens, NO_SPRING (summer, autumn, winter), that disable them from appearing. So you could have castes: "bunny" for summer and autumn, an "white winter bunny" for winter, and a "easter bunny" for spring. But I don't know how much people will notice, considering the large amount of animals in the game. The chances that your few modded ones will show up are rare.
Active seasons are of course also a part of entities. You could have the hogfather or christmas elves appear in winter, with their "sled" wagon, being pulled by reindeers or hogs. ^^ Same for the other seasons. Spooky Jack Skelington coming by in autumn to peddle pumpkins and skulls.
Overall: Yes, you can kinda do this, but most options would affect the entire season, not just a few days/weeks. I would assume that dfhack scripts would be much better options for this. Big popup, pausing the game, saying "THE HOLIDAY OF X HAS STARTED!" and for the next week all plants grow instantly; or dwarves get free alcohol, or happy thoughts added. All undead put to rest, that sort of thing.
I had an idea to somehow make seasons more seasonal by somehow incorporating aspects of holidays into the game. Although holidays are a cultural thing (and therefore would be handled per civ during gameplay), I was trying to think if there was a way to incorporate holidays into the gane through a mod.
This could take the form of certain characters/creatures showing up at certain times or somehow adding holiday specific items for use during their holiday season or specific seasonal crops/food etc. Does anyone have any thoughts on this? Any ideas on bringing holidays into the game? The holidays wouldn't have to be the modern holidays, but take inspiration from them (e.g. fall: harvest festival, spooky festival, feast, winter: gifts, decorated trees, etc.)
Bad idea for mod. This will not holiday for dwarves. Add suggestion to forum, this will be in future update.
If people enjoy doing mods, why not let them?