There are references to many celebrations in Star Trek. Since most of them don't have dates they could put them any where they wanted. In line with real holidays so the players could take part when they are off of work or out of school.
If STO is going to put in holiday events, then they should be Star Trek Holidays like:
First Contact Day - April 5
Day of Honour
Traditional holidays don't seem to be celebrated on Star Trek so they shouldn't be celebrated in STO.
These are definitely holidays that STO should be celebrating.
I wouldn't mind Earth featuring a holiday from each of the major religions. Or even if not specifically religious then at least a new year's celebration on earth, perhaps making SFA textured with snow on the ground and put in snowmen and snowball fights by the cadets that you could join in I know that Cryptic can do snowballs... I got them as a bug during season 4 testing :p in place of certain rifles.
Andoria would make for a good Christmas area since it's already snowy and they could just plop in some objects.
I would love to see special events specific to each of the founding members of the Federation. Perhaps a Surak day on Vulcan?
Also perhaps some somber events, such as a day of remembrance for those lost in the Romulan War or the Dominion war that could coincide with our own Memorial Day.
That's my fear, if not expectation, as well. Though what is underwhelming for me may not underwhelming for everyone. Stuff like "gift packages" with random stuff or whatever you can buy on the C-Store and get from Santa Qlaus daily will underwhelm me, but it may be appreciated by some for the fun diversion it is.
It couldn't hurt if Cryptic would develop some "In-Game Holiday" Events that could happen every year.
Q'onos: Kahless Memorial Day
Rura Penthe: Summer Festival (the only 3 days in the year where the snow retreats far enough to see some land?)
Gandala Station: Klingon Liberation Day
Andor: Federation Founding Day
Earth: First Contact Day
Vulcan: IDIC Contempation Day
Bajor/DS9: Bajorian Gratitude Vestial
Drozana Station: Comet Day or Ferengi Alliance Formation Day
Some events could coincide with real world events (Thanksgiving <> Gratitude Day, Christmas = Rura Penthe Summer Festival and IDIC Contemplation Day, Silvester = Comet Day).