From the game's encyclopedia
An Activity where rulers and courtiers in a realm gather to celebrate... one thing or another.
Participants will lose Stress and increase their Opinion of the host. There may also be any number of other consequences from bringing that many people together.
A Ruler will invite their Liege and their Vassals (along with various Courtiers) to a Feast.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Feast is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "An Activity where rulers and courtiers in a realm gather to celebrate..." It is part of Activity. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.