The word bonenkai is represented by three well-known kanji: forget, year, and gathering. It is a ceremony held across Japan at the culmination of every year, meant to celebrate the completion of another successful orbital period.
These parties are held amongst coworkers, involving heavy drinking and overnight trips. This year, all of the teachers from one of my junior high schools are going to stay overnight at a ryokan, which is something akin to an old-fashioned Japanese hotel. I was invited, but I will be partying in Tokyo at that time. According to this very-hard-to-read Japanese website, bonenkai go as far back as 1400, the Muromachi Period of Japanese history.
Japanese parties are an opportunity for coworkers, who must always act formal around each other as dictated by the unspoken laws of society, to shed their business facades and commingle in a more informal manner. These types of parties are always organized with a set fee to cover expenses, ensuring that everyone pays the same and receives equal treatment. It is a serious attempt by colleagues to make sure everyone is included, one of the many things I appreciate about Japanese culture.