Christmas day in Japan is known as a festival, a time to spread happiness rather than a religious celebration. Christmas Eve is often celebrated more than Christmas Day. Christmas Eve is thought of as a romantic day, in which couples spend together and exchange presents, in many ways.
No matter what is your religion or belief, people celebrate Christmas happily in Japan, although they may be don't know what the origin of Christmas is. The big corporations do the main decorating. They light their buildings and the trees.
Japanese Happy/Merry Christmas is 'Meri Kurisumasu'. Parties are often held for children, with games and dancing. Japanese Christmas Cake is a sponge cake decorated with trees, flowers and a figure of Santa Claus.
what is the special food during Christmas in Japan ? The answer is Chicken. It is the busiest time of year for restaurants such as KFC and people can place orders at their local fast food restaurant in advance.
The traditional Japanese Christmas food is Christmas cake, but it's not a rich fruit cake, but is usually a sponge cake decorated with strawberries and whipped cream.
Sending gift and exchanging Christmas gifts is a custom till today in Japan. For references, here are some famous Japan Christmas Gift :

Japan Christmas gift black two tiered box

Japanese Christmas Cake. It's common to eat Christmas cakes on Christmas Eve in Japan. Typical Japanese Christmas cakes are round sponge cakes with whipped cream and fruit toppings. To bake a fluffy sponge cake, make sure to whisk eggs very well.

Santa Claus Is Coming To Town and he comes bearing gifts! This basket is a great gift for a baby's first Christmas. The red bodysuit has a Christmas tree on the front with the words "My 1st Christmas" on it.

The KFC 「パーティ・バーレル」 (“Party Barrel“). KFC is one of the busiest place during Christmas besides cake shops in Japan

Japan Christmas Cards Geisha and Santas

Japan Christmas Cards

With best wishes for christmas and a happy new year

These are really cute animal stickers and Christmas greeting card By Kamio Japan. It's called "greeting card seal". The texture of these stickers are sparkle. There are cute felt bear plush, felt rabbit plush, felt Christmas cake, gingerbread man, candles, candy, Christmas presents and etc
Watch We Wish You A Merry Christmas - Japanese Christmas Song video here