closing party; closing event of a festival
Typically refers to the final celebration or ceremony at a school festival (学園祭) or similar event, often with performances or a wrap-up gathering.
See also: 学園祭
Fireworks were set off at the school festival's closing event.
The executive committee is taking the lead in preparing for the closing party.
学園祭 is the school festival itself; 後夜祭 is specifically the closing party or ceremony that concludes it.
前夜祭 is the celebration held the night before the main event, whereas 後夜祭 is held after it ends.
Compound of 後 (after) + 夜 (night) + 祭 (festival). The term likely originated in school festival culture to describe the final evening event.