I'm pretty sure that when we were at the same technology level than the one that's depicted it DF, events like harvest festival or veteran's day meant something and were celebrated.
Anyway, I thought a bit about that really nice suggestion and that's why I thought:
Parties should be implemented so the more the player put work to help the dwarves attend them, the less it will have a negative impact on the fortress. Eg: The player never bothered to make meeting rooms? The dwarves will spend way more time finding a place to celebrate. Or, they will have a small negative thought ("Was unable to celebrate XXX properly this year").
Also, maybe the parties could be prepared by non-working dwarves? The usual suspects will include nobles (organizing the whole thing), children (preparing rooms, tables, or even decorating with unused trade trinkets), and the standard peasants / haulers (bringing food and booze to the nearer stockpiles, or directly on some tables). Maybe even "On break" dwarves could participate, after all preparing is part of the fun.
This way, the parties and festivals will still be impressive and lively, but won't delay your Giant Canal or your Great Defense Wall at its most critic point.
Soldier will only be allowed to attend parties if they're off duty, of course. But expect bad thoughts for the lone marksdwarf that's forced to wait on the snow while everyone is celebrating New Year’s Eve inside, nonetheless.
Commemorations and religious event could also happen at the site of the original event or near a symbol of the deity, respectively. Near the banks of a river (which could lead to the “Carp incident” commemoration lately, hehehe) for a deity related to rivers, for instance. Or every 3 years after a big battle, the dwarves all gather on the (roughly defined) battlefield where 6 dwarven soldiers and 38 goblins perished in what is now known as Dakust's Last Stand, and maybe the mayor will make a speech or something.
Kind of celebrations I thought of:
- New Year's Eve / Fortress anniversary (could be a different event if the fortress was able to start at a different date). Two possibilities:
Intimate celebration => Everybody stop working the 25 (or 27, or 30, it'll need smoothing) of late winter. Not that much preparation. Families and friends will gather in their rooms and make parties of their own. Groups of dwarves will choose the nicest room they have. If they have no rooms and still live in a big barrack, then they'll probably seek an isolated place, or the usual "was unable to celebrate properly" thought. Dwarves with no friend or relationships whatsoever will either party alone on their room (probably with a bad thought) or gather in a public meeting room to party with other friendless strangers, possibly starting new relationships. Before the first migrants wave, the 7 starters will all party together, regardless of the relationships.
Or…
General fiesta => Everybody stop working the 25 / 27 / 30 and head to the largest dining / meeting room. Huge preparations, maybe one month long. Everybody talk with each other, and gain social skill. Then when it's 1st of spring, maybe they all do something special? (All drink at once with a special mug, or do something related with the specialty / symbol / name of the fortress)
- Harvest festival: well, since dwarves cultivate all year around, it's a bit hard to tell when they should organize it. Maybe at the end of Fall? It will be a low-scale festival, maybe only celebrated by farmers. Dwarves that need to eat will try different products, or maybe only meals made of plants.
- Foreigners festival: A special event that could happen every 2 years or so, on mature fortress (and with sufficient import / export). Delegations from humans / elves / whatever race arrives at the fortress, and a festival promoting the culture of this race is held, with traditional dishes, music, etc.
While it will be awkward to have an elven festival inside a dwarven fortress, it seems fun and after all, they aren't allies for nothing. Dwarves could also try to show respect or admiration for the guest's culture, maybe by suspending all woodcutting jobs when the elves are here, or by trying to dress human clothes? At the end, there could be an exchange of gift between the local ruler (expedition ruler / mayor / baron / king etc.) and the chief of the delegation, each one offering a typical craft of his nation of similar value, like:
“Urist McBoss, Mayor has offered a +<<Silver gobelet>>+ to the human diplomat!
Urist McBoss, Mayor has received a *Steel crossbow* from the hands of the human diplomat!”
or:
"Urist McGreatboss, Count, has offered a +Bronze luth+ to the elven diplomat!
Urist McGreatboss, Count, has received a ¤Feather tree table¤ from the hand of the elven diplomat!"
With caravan arc and the like, you could also try to send a delegation once in a while to allied civs, with a gift and some of your best craftdwarves, to get similar results.
- Love day (equivalent of valentine’s day) : Married couple will spend some time together, and single dwarves will actively search for their other half. Maybe also a bonus roll for "breeding", possibly leading to a baby boom some months later. A very low-scale celebration, with almost no impact on the fortress activity.
- Children could also throw a light celebration for their birthday, with their friends, simbling and parents (again, good thought if nice birthday, bad thought if the workaholic parents didn't thought of wishing him/her a happy birthday).
I also saw a lot of good ideas above, like the celebrations or commemoration for disasters, artifacts-related events, military victories etc.
I think that kind of stuff could add a lively and emotional feeling to the fortress, as well as having some benefits (using otherwise useless items (instruments, mugs etc), providing jobs to children and nobles, strengthening the influence of concepts like deities or allied civs in the game…). Will you mind if I had a suggestion to the eternal voting list and link it to this thread?