December marks the loveliest season in most Christian nations. It's the season for sharing. However, being a Buddhist and Shintoist nation, Japan spends December for partying, rather on shopping.
Bonenkai means "forget-the-year party". It is the wildest among all celebrations and parties in Japan where the people, especially the salary men stays up late at night in pubs and bars throughout December.
There are also bonenkai held in school where students, parents and teachers celebrate and have fun via bingo, party games, food, drinks, and song/dance numbers. Some of the bonenkai also involve exchange of gifts and reminiscing the ending year.
A welcoming new year party is also held in January called "Shinnenkai", celebrated to wish good luck and have another prosperous year. They usually say “Akemashite Omedeto”, which means “Happy New Year” in Japanese.
Whatever you call that party in December, cheers for the coming year! Kampai!