Shopkeepers give discounts (perhaps treating you as if you had a higher Charisma than normal, to avoid the issue of selling items for less than they would buy them for) on Black Friday.

When a bones file gets loaded, a copy of it should get made and stored somewhere which will prevent it from being loaded in another game - until Halloween. On Halloween, any level that is eligible to generate bones and for which a bones file exists will generate one from this special pool.

On Halloween, ghosts should be able to generate randomly.

Add the following events on Halloween:

• The normal “midnight” effects should begin at 6 PM.
• Intelligent monsters may generate peaceful and with candy bars, which they will give to you when you chat to them. Or possibly, this only happens with monsters that have “homes” such as shopkeepers, priests, and peaceful Minetown inhabitants. (Not specified what happens if a monster generates with a candy bar, but you save the game and come back to talk to them once it’s no longer Halloween.)
• Other monsters have a higher than normal chance of generating with attack wands.

Various proposals for things to happen on St. Patrick’s Day:

• Leprechauns should get buffed. There are various ways this could be done; probably the easiest is giving them more starting gold but also increasing the damage of their attacks.
• Leprechauns could spawn with potions of booze (always for individual leprechauns, low percentage chance in leprechaun halls). Ideally, the potion should be green.
• When random gold piles generate, leprechauns should frequently spawn on them.
• Random gems should have an elevated frequency of emeralds and green worthless glass.
• If the game is started on St. Patrick’s Day, the potion of booze is always shuffled such that it’s green.

On Easter, eggs appear in various bright colors instead of plain white.

On April 15 (tax filing day in the United States), you are charged a sales tax in shops.

On April 20, all petrifying attacks of monsters have the same effects they do on a new moon, i.e. making stoning more likely.

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.