Hi!
I've only spent a single Christmas here so far, but from what I've experienced and from what the Japanese people I've spoken to have said; the New Year and Christmas holidays are essentially flipped. i.e You spend time with friends on Christmas and then New Years is a quiet time with the family. However, it's important to note that most places are still open as per usual on Christmas; so it's much less of a formal holiday. Christmas dinner is also the night before Christmas, whereas I'm used to having a Christmas lunch on the day with family (and it being warm).
What we did for Christmas though was have a big dinner with a lot of people in the share house, everyone bought a plate of food and I cooked a couple of chickens outside in the snow. There was also a small gift giving thing too where people spent ~1000 yen on a gift to be given at random. There were lots of ads for KFC on the trains, but nobody that I knew was doing that for Christmas.
We didn't have "Christmas cake" although my idea of Christmas cake is a heavy brown cake with a lot of fruits in it, all of the cakes here are mostly spongy, but anything that looks "Christmasy" counts as a Christmas cake. As for the actual day, nothing really happened... it was kinda just a normal day where a lot of people would use the time to go and spend time with their friends/boyfriend/girlfriend/partner.
New Years was a little different, there were no fireworks. However we all watched the new years broadcast on NHK and ate shabu-shabu together. Lots of alcohol too! I had a bottle of sake that we all drank together on the new year as it was counted down on TV, a special year of the dragon bottle! Once it was 2024, we all visited a shrine, prayed, bought a good luck charm and fortunes, then went back home. On the day itself, most people spend it at home as many places were shut.
Oh, forgot to mention there's lots of Christmas songs playing at Christmas time, they're almost always in English except for a select few. One that sticks out is 赤鼻のトナカイ akahana no tonokai or "red-nose reindeer" (lit).
I hope this helps paint a picture for you!
