Christmas isn't a thing in the world my two MCs inhabit, but any culture that deals with short, cold days in winter tends to develop some sort of solstice holiday or observation, because your options when staring down the barrel of a cold, dark, uncertain winter are twofold: 1, crushing depression or 2, party.
Adi's culture, in the small woodland village of Silberyn, worships a pair of twin deities who balance the absolutes of reality. Winter belongs to the brother god, Valkus, who also reigns over night, the moon, death, and many other concepts. (His sister, Edhari, in contrast, is the goddess of summer, day, the sun, and life.) Winter solstice celebrations are often full solemn remembrances of ancestors and the recently deceased during the day, and bonfires, song, and feasting at night. They hope to both give Valkus his due by acknowledging his roll as god of winter and death, and also encourage Edhari, goddess of light and life, to return swiftly. Even though Adi has.... very mixed feelings about her home and the gods that rule it, she probably still observes these traditions on her own.
Koreal's culture, on the other hand, is much less of a hamlet in the woods-- he's from the much larger city of Mir'Aran, a wealthy, powerful city-state and trade hub. They're much less dependent on nature for their immediate survival, so the winter solstice is mostly an excuse to party. One of the biggest events of the season is the casting of the Winter Flame. See, Mir'Aran's infrastructure includes elevated basins dotted around the city, all of which are magically tied to a grand cauldron in the courtyard of the royal palace. During the summer, the cauldron is full of fresh water-- which causes the other, smaller basins to fill with fresh water, too. It's functionally a public water fountain system for the hottest months of the year.
During the winter, though, the archmage makes a big show of casting a fire in the grand cauldron, which again translates the effect to all of the basins around the city, so that no one is without a source of warmth or light during the darkest, coldest times of the year. While he's probably bored of the pomp and ceremony of it, Koreal has probably attended at least a couple of Flame ceremonies in his life, and anyone who grew up in Mir'Aran is probably quite fond of the sudden infusion of light into the city.
...although Koreal is just as likely to say 'nope' to snow and darkness and go winter in the tropics. Because wizard.
