There are 2 two-week celebrations that we know of so far, for what it's worth, and they are vastly different. I would assume the races (especially ones not morally aligned) will have different holiday schedules.
The first is from the Dark Myr, known as "The Day of Testimonies", which 'celebrates' their transfiguration by the goddess Syronai into creatures who could survive on Terminus. It runs a week before and a week after the demarcated day. Honestly it's a lot of reading of testimonies of how much it sucked to come to Terminus (hence the name), so I bet the Dark Myr parties will be super fun.
The other is from Elven tradition, and is the chief holiday of their calendar. It celebrates the survival of their Mother tree, Heartseed, despite it being burned severely by the Ravaging Lord. It too runs "a week before and after" the particular day. Seeing as this particular event spawned two factions of Elves, I assume it will become a particularly important part of their holiday traditions.
edit: I bet you dollars to donuts the schedule for holidays is not finalized. It seems like something that, while important to the lore of each race, is also something whose actual celebration in game will not manifest till later art passes. Wouldn't surprise me in the slightest.