Happy Day after July 4th! Hopefully everyone got all or at least part of the day off to stand outside and sweat by a BBQ watching fireworks :)
Thinking about the reason for the season, as it were, got me thinking about holidays in other worlds. People as far back as we can figure have had their own celebrations. They've had special days that commemorate events, nature, the seasons, deities, and whatever else was important to them. It follows, then, that your created world could have the same thing!
Due to scheduling, it's not always possible for a LARP to set aside a certain date for your holiday. But there's no reason why your world can't have a, say, mid-summer festival. Or really any type of festival that commemorates a certain event around a certain time of year. PCs, you could even start this off by recognizing a previous event you were a part of (Happy Day of the Dead Lich everyone!). Then keep doing it at the same time every year.
This can add a lot to the in-game atmosphere. Holidays are something that gives a people an identity, as what you celebrate says a lot about you. Look at Americans - we've got quite a few days that others don't specifically share, like 4th of July, Thanksgiving, Memorial Day, Labor Day; and that says a lot about what Americans have valued as the holidays were created. Some days may be wild, some may be somber, but it can really help build your world and your NPCs. If you only run a few events a year, these holidays could even be the reason for the gathering. Rather than referencing the last "market day" or whatever your group uses, you could instead talk about what happened at your harvest festival, or last Dragonslaying Day.
As plot, I've always loved using holidays (maybe a bit too much) as inspiration for mods or other encounters. If anyone remembers the 3-legged turkeys at Ashton around Thanksgiving, I apologize :) But holidays can give you great ideas for mods where you need them. In spring, maybe some potions at your Festival of Love got out of hand. In the fall, maybe different groups need help harvesting. After going to a lot of events, they can run together, so giving something a theme can make that game memorable.
You can also use it as a reason to have games, contests, and other events. Staff doesn't need to set it up, they can work with an interested PC to help it come up. These can be contests of skill using in-game and out-of-game skills. I've found it's good to have both, so that people who are good at one or the other can still participate! And don't leave out non-martial talents like art, baking, or performance. This is fun for the players and also helps Staff take breaks as the games are going on so they can concentrate on other mods knowing the PCs are occupied.
I will say that this sort of all-out fair can't be done too often. Every one of your events can't have contests of strength, as that can get boring - unless, of course, you've got a fun plot reason. A town of barbarians may regularly see who's stronger to determine leadership or something. This could even evolve into a ritual of some kind - but I digress :) In general I think the character that was the strongest at the June event will be the strongest at the July event, so in LARP time doing this once or twice a year makes more sense. I think the specifics about running a game-day festival could be a whole new post, so I won't go into it more.
You can also support the theme in other ways. Have someone make a special Midwinter treat that you only make that time of year. Give out little trinkets that represent the holiday. Come up with traditions for your NPCs to mention, and even incorporate them in game. Example: If you have a day of remembrance, bring in one of your NPCs to lead a candlelight vigil, make a speech/toast, add new names to the banner of the fallen etc. This specific example double-dips, since it goes back to the post about remembering fallen characters (see what I did there)! :)
But what do you think? Does your game have any regular festivals? How can it work for you?