I am thinking to go to Bergen during Christmas holiday on 25, 26, 27, 28 December. I have already checked that train and hostel are available, however I am worried if everything will be closed since it's holiday and it will be very limited for the places to visit.
Both December 25th and 26th are public holidays in Norway and also on the 28th (being a Sunday), all "regular" shops will be closed. Smaller grocery stores and souvenir shops may however be open.
I checked the opening hours for a few of the most popular museums in Bergen and none of them mention special opening hours for the week between Christmas and New Year's Eve on their web pages, meaning that they are open Tuesday to Sunday (most museums are closed on Mondays). I do however doubt that this is correct and assume that the museums are closed at least 24th to 26th of December.
Lots will be closed. The Pepperkakebyen will be worth checking out, and you can still go on hikes around Bergen (the advent candles will be lit up at Fløyen). Also, look into going to the Lysfest if you are here in time.
The biggest thing to consider are the grocery markets. Bunnpris usually stays open when others are closed, but I believe even they have limited hours on Christmas. Most stores post their holiday hours (and which stores will be open) on their websites.
Here are links to the main stores in Bergen:
In addition, there are several stores in the Coop group in Bergen (Coop Extra and Coop Prix).
Check when it is closer to Christmas for "juleåpningstider" (Christmas opening hours).