First of all, In Japan there is no days off on Christmas.
The 24th is usually considered Christmas. In reality there is no official day in Japan, so most people consider 24th to be special. It is the special day where people may participate in a variety of events. Christmas is a fun decoration time with small events that take place during that time. In the reality, its a time where companies have a chance to make money.
In Japan, サンタさん SANTA SAN (Santa) still comes but he usually only gives a gift to children or couples. And the biggest difference is you only get 1 present.
クリスマスケーキ KURISUMASU KEEKI (Christmas cake). In Japan, we eat cake on Christmas. This is probably the biggest part of the holiday in Japan. When people think of Christmas in Japan, they think of Cake. People spend big bucks on cake the cheapest cake runs at around 20$ and people spend anywhere up and over 100$, just for a cake. Cake is needed.
If you don't know, Japanese people love their sweets. Foreign people are stereotyped as loving sweets, but Japanese consume way more sweets. There are cake shops and special cookie shops everywhere.
ケンタ KENTA (Kentucky fried chicken) is another thing that is popular on Christmas in Japan. Trying to get chicken on this day can be impossible for some if you didn't make a reservation. You will see people lining up for a piece of chicken.
NOTE: The menu changes on Christmas as well, You can't get regular crispy chicken. It is Christmas prices and Christmas style BBQ chicken piece.
Grocery stores also start selling Christmas plates of special foods that are common at Christmas. Which are usually Chicken and vegetable bento boxes with a nice Christmas design on the box.
Outside you will see イルミネーション IRUMINEESHON (lights) around Japan. Japan goes over-board with the whole Christmas lights thing. It is almost like a festival or a competition.
They can be found outside regular malls and stations, but there are designated areas where huge demonstrations are.
When it comes to decorations, almost every store and shopping mall will be full of Christmas themed stuff. Some people decorate their house on the outside (not usually the inside). Even Christmas trees are put outside. You can see them on someones front porch.
Love hotels (hotels where you go to have sex with your bf/gf at) are usually packed around Christmas time. It is seen as not only a kids holiday, but a couples holiday.
For this reason many theme parks such as Disney land have special Christmas time themed stuff. You will see lots of nice commercials for nice date spots for Christmas.
As for Christmas songs, You know its Christmas time when the number one
So enjoy Christmas in Japan.
With your one present, Christmas cake, KFC and you can sing Last Christmas. You can do it all at once even. Why not go to Christmas Karaoke?
Heres an Extra video about how My christmas was in Japan, and video that I took around Japan on Christmas