This one has me wondering for a while.
Fantasy. First world that comes to my mind when I think about serious fantasy (to exclude Pratchett at the doorstep, sorry man! :*) would still probably be Forgotten Realms thing.
Maybe because it's the first one I ever really got into, maybe because it's definitely one of the biggest (created by a horde or people not one).
Thing is, pretty much everything there is custom. Custom customs, a grand multitude of races, barbarian tribes, shark-people, everything and anything.
Then there are plenty of other fantasy worlds that seem completely alien and foreign to our own. And they work for fantasy, they really do.
So the question is: would a world playing dress-up work you as well?
For example, let's go with the season. We have Christmas. Assuming the most common way of celebrating Christmas.
OU: Christmas tree, presents underneath, whole family meets and eats a ton singing carrols.
AU: Underwater society of mermaids. They find a huge dead jellyfish, take to their home-cave and then put shining deep-sea fishes on its branches. Then proceed to feast and give each other gifts made of seaweed. It's a celebration of... um... I don't know, let's say the day their god made water transparent so they could see the sun. Everybody is full of love towards the entire world.
Or less different examples, cause I kinda zoned out with the jellyfish:
Birthday. A group of forest elves meeting together in a tree house to celebrate someone's birthday by giving them presents and singing a song. Don't forget the cake.
New Years. Pirates at the ship take cannons and fire flares into the air for colorful firework-mock-up to celebrate the coming of a new year.
Chance that there would be exact same customs in a foreign fantasy world and our own are slim to non-existent, of course. But does it bother you? It never bothered me, yet I see that it is not the most common practice when it comes to fantasy.
It reminds me of playing a dress-up. Theoretically it looks like a dragon now, but you still see a good old grizzly bear underneath, because some fur is sticking out and it growls instead of roaring.