On Friday the 13th, kittens, housecats, and large cats are referred to as “black kitten/housecat/large cat”.

During Hanukkah, oil lamps should burn at a very slow rate (1/8 the rate, unless this is unbalanced) so that they last a lot longer than usual.

A bag of tricks occasionally spits out a candy bar instead of a monster (or perhaps along with / carried by a monster), but only on Halloween.

On Easter, randomly generated user-nameable fruits will generate as “easter egg” rather than slime mold or whatever the user has customized them to be. Despite the name, it remains the exact same item in game and does not break vegan conduct.

Calculating whether a given day is Easter is a complex but solved problem; see here.

On March 15th, your first minor consultation from the Oracle will always be “Beware the Ides of March.”

Extend the Advent Calendar patch to include Hanukkah gifts: behind the same appropriate doors depending on the date in December, you can find candles and potions of oil. If mundane candelabrums are added, possibly add those too.

On Valentine’s Day, succubi (or any female seducer) steal candy bars you are carrying, and incubi give you candy bars.