I have a theory that desserts that are only on holidays really sort of suck or they would have broken out of the holiday and are just getting by on the nostalgia factor - how are these mustards you talk about?
Also can you name some of them? I guess I can always take the train over to Malmö in a couple of months to pick some up to test the quality myself.
on edit: fixed typo
People have time off on holidays, and families all get together increasing the available labor pool. Accordingly holiday foods can be more labor intensive than foods eaten during the rest of the year, which is an alternative explanation for why some foods only appear at holidays.
As another commenter mentioned, egg nog is one. Home made egg nog is a fair bit of work, doesn't keep for long (well depends on if you pre-mix the booze I guess), and is calorie dense enough that eating it year round would be a mistake.
Heck home made pies in general.
Also certain foods are a lot of work to get setup, but they scale up very well. It is just as much work to mull a little bit of wine as a lot of wine. Holiday cookies, kind of both. If you are rolling out cookie dough mine as well roll out a lot, and if you have lots of kids around, free labor to do the decoration!
As an aside, store bought eggnog is not good, if you don't like eggnog, try home made, you can adjust consistency, flavor profile, and sweetness dramatically. I never liked store eggnog, but on a whim I made it at home once and it turned out grand.
If it’s Christmas mustard you’re hoping to find, simply search online for ‘julsenap’ before your visit.
Regional variations will more likely turn up at Christmas Markets (julmarknader), but even normal grocery stores will have more to choose from at that time of year.
An unusual tradition - a hangover from a time when people used mustard for its medicinal properties - is that chemists sell julsenap at Christmas.
There are literally dozens of varieties to choose from nowadays - some of them including unconventional flavorings and spices. Here’s one example (not a recommendation! I’ve never tried this - just googled julsenap and this was one) which includes chili and lemon: https://dellback.se/hovsenap-julsenap-chili-citron
https://www.swedishfood.com/swedish-food-recipes-starters/13...
Ikea generally sell mostly their own brands, at least here in Sweden.
But some of it I agree. Never learned to enjoy the traditional Norwegian food. Now it's "fårikål season", which is basically just lamb+cabbage boiled for far too long. Or "smalahove" where you eat the sheep's head. Christmas is the samme. Take ribs of the lamb and damp with salty water. Might be lamb I don't like, though.
My main exhibit for this is the Danish ris a la mande which is during Christmas, it tastes great to me (nostalgia) and Danes if told it's crap will defend it. But foreigners don't taste the goodness, and really if I think about it when eating I start to think: "you know, a tiramisu is better, so is risengrød for that matter, this stuff sort of sucks but I like it."
Ris a la mande also has a specific Christmas tradition associated with it (you have to feel with your mouth for the whole almond in it, and if you find it you get a gift) so doubling down on "it's not taste" explaining the love.
Many are originally seasonal for logistical reasons: before refrigeration, food had to be eaten close to harvest time or preserved in some way. So you have pumpkin dishes in fall because that's when the pumpkins are ripe. Likewise tomato dishes in spring.
Winter holiday foods often get that association because they are preserved. We ate them then because we had nothing else to eat since the harvest is over. That gives you eggnog, cured meat, jams, etc.
Once a dish gets a strong association with a certain time of year, I think it tends to hold it because the association is part of the enjoyment. Sure, I could have a mug of eggnog in June, but it wouldn't be as magical.
I have a counter-example.
I love peppermint stick ice cream, and it is impossible that anyone might not also love it. :-)
When I was a kid back it was one of the always-available flavors at the nearby ice cream shop. I don't know if was just luck or not, but now it seems peppermint stick is available only around Christmas, and even then I often have a hard time finding it.
My personal favorite is the polish mustard with honey, sometimes I just eat plain bread with it if there's no pashtet around to go with it.
In various parts of Europe there's a strong tradition of very geographically-specific types of mustard - often differing greatly in texture, strength, color and taste in neighboring towns or regions.
Also mustards can be seasonal - here in Sweden there are many mustards that are only available at Christmas, for instance - not to mention that people make their own mustards (also usually based on regional preferences).