During Winter, Japan is very cold and snowy in the nothern areas. However, the weather will not prevent you from doing your activities and visiting many places. You still can enjoy the beautiful winter going to the famous Sapporo Snow Festivel, strolling trhough a christmas market, or event doing some skiing, eating seasonal Japanese foods, and seeing the christmas illuminations.
Also, there's the possibility to enjoy the Hatsumōde, celebrating New Year like a local in a shinto shrine.
Here's a small list of some of the best things you can do when you visit Japan during the winter. Please keep these in mind when planning your trip.