Much of the time, they're a chore I do so my sims won't have negative buffs. I strip them down to make it easier, though: Love Day is just any romance behavior, Thanksgiving is just being thankful (though in some saves it's called The Big Game and involves watching sports and eating), etc. I do occasionally get into it, usually when my playing a Sim's holiday coincides with the holiday season. Lately I'm playing with integrating Stardew Valley events and TS4, so in some saves Winterfest or whatever is Feast of the Winter Star, and I play the appropriate music during the celebration!
