taken from Japan Related Information
Christmas in Japan is quite different from the Chrismas celebrated in most countries in which the population has a large percentage of Christians or a Christian heritage. Only 1/2 of 1% of the Japanese population is estimated to be Christian, with the majority of Japanese being tolerant of all faiths: Buddhism, Christianity, Shinto, etc. In spite of this, the Japanese are great lovers of festivals and celebrations, including Christmas.
December 25th is not a national holiday in Japan, although December 23rd, which is the birthdate of the present emperor, is. Although it is not an official holiday the Japanese tend to celebrate Christmas, especially in a commercial way. The Japanese celebrate Christmas Eve by eating a 'Christmas Cake' which the father of the family purchases on his way home from work (or his wife does in the case where he has to work on Christmas Eve). Stores all over carry versions of this Christmas cake and drop the price of it drastically on December 25th in order to sell everything out by the 26th. This has resulted in a rather interesting expression in which young girls are referred to as a 'Christmas cakes': marriageable until their 25th birthday and requiring heavy discounts to get married after their 25th birthdays.
Christmas Eve has been hyped by the T.V. media as being a time for romantic miracles. It is seen as a time to be spent with one's boyfriend or girlfriend in a romantic setting, so fancy restaurants and hotels are often booked solid at this time. It is often also a time when girls get to reveal their affections to boys and vice versa. Because of this, extending a girl an invitation to be together on Christmas Eve has very deep, romantic implications.
Relevance to Urusei Yatsura
Just as New Year stories show up in the series once a year, Christmas stories seem to as well. In particular everyone is asked to climb the giant Christmas tree at the Mendo Estate, or eat their way through a giant Christmas cake filled with bombs.