We celebrate on the 24th, Finns generally celebrate everything on the Eve for some reason.
The 25th and 26th are also days off, that's when you can play with your presents and stuff. In families with kids, Santa often comes to visit on the 24th, usually in the evening. He hands out the presents to the recipients, reading the tags on them. For many, going to sauna is a tradition, and visiting graves is another. When I was little, we had quite a strict schedule, visiting the graves, getting everyone showered/sauna'd, and eating the requisite number of times before it was Santa time.
At least in my family, the "big dinner" is on the 24th, although we do eat the same foods for three days. The most traditional main course is ham, although turkey, fish and vegetarian options have become popular in recent years. Also there's (at least) two kinds of Christmas hams, and often a family will have a strong preference, the other kind of ham is the wrong kind.
We also eat different casseroles (potato, carrot and turnip casseroles are the usual ones), various kinds of fish and meat, salads (one traditional one is rosolli, made with beetroots, carrots, onions and herring at least)... Ginger bread and pastries with a plum jam filling are popular desserts, chocolate of course and fruitcake is another. Oh, and at least in my family, the traditional Christmas Eve lunch is rice porridge.
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