Something that often doesn't happen in science fiction is holidays, yet most of us arrange our lives around major times when you do certain things partly because you've always done them and partly because that's what people do at that time of year. It gets really weird when something like Christmas, which blends a whole bunch of winter solstice festival traditions into the Christian message, and has since kind of spilled out into a month or so that encompasses a bunch of things that are mostly based on the pagan festivals... Er, where was I? Oh, yeah.
It gets really weird when Christmas, in its form as a predominantly northern hemisphere winter festival, gets transplanted more or less intact to the southern hemisphere, with all the midwinter traditional trappings attached. Of course, that's exactly what I grew up with, so I never saw anything odd about fake pine trees and fake snow or the big fat guy in the red suit with all the fur everywhere being right in the middle of summer. In fact, since I moved to the US, I've been consistently weirded out, every year, by the novelty of Christmas in winter.
Then there's the holidays I simply don't 'get' because they were never part of the Australian tradition, like Thanksgiving, and the ones I miss because they're not part of the US tradition.
Now imagine all those traditions on a space station with no seasons at all. Or a planet where the year just plain doesn't match our calendar. How weird is that going to get? (Hint: 'very'). How about praying to Mecca when the direction you'd need to face would put have you kneeling on the wall? (Okay, not so hard if there's no gravity, but the prayer mat would have to be stuck on, then moved each time because the relative positions would keep changing)
Of course, people will adapt. They'll adjust their traditions a bit, put Santa in red board shorts and thongs (the footwear. The thought of a big fat guy in thong underwear is just...EW), and the absurdities of singing about snow and ice and all when it's hot enough to cook eggs on the pavement just won't occur to them because that's how it's always been. Maybe some of the people will have scales and be a bit bewildered about the way these soft-skinned mammals do things, but hey, it's kind of fun anyway, and we can get drunk on their booze and they get drunk on ours, so let's party! (This does rather assume that the aliens in question met a group of Australian or similarly-minded explorers, whose sole criteria for 'is people' is 'shares the booze').
Or maybe it will go the other way, and people will jealously guard their traditions and refuse to change one iota no matter what. I'm not sure I know anyone who's that rigid, but it's happened in the past, so it can certainly happen again. Actually, my money (and I come from a land where they'll cheerfully bet on two flies crawling up a wall) is on 'yes', and 'everything in between'. One thing history tells you, quite loudly if you don't stick your fingers in your ears and go 'neener neener neener I'm not listening' is that if it can be done, it will be done, usually with some kind of sexual fetish attached.
What do you think future holidays will be like? The only answer I don't think is legit is that they won't exist. They will, because marking certain times as "special" is part of how we personally distinguish the passage of time. It's never 'four Novembers back', it's 'Thanksgiving four years ago'.