An enkai is a work related drinking gathering. It depends on the workplace, when it decides to hold an enkai. It's usually after a big project or when someone's either welcomed or retiring or moving. I've only been invited to one so far. In a school, they usually hold one at the end of a semester. Most Japanese companies hold about 3 enkais a year. Kangeikai (Welcome party), Bonenkai or Shinenkai (For either the end of the year or the beginning of the year) and a Soubetsukai (Farewell party). Seating tends to be by lottery. One thing to remember is to fill your coworker's (especially senpai or boss!) drinks with two hands or order them a new drink. Never let their drink be empty! or until they don't stop you...