Game concept · Advanced Activities
Cultural Festival
The game also links it as Culture Festival, Cultural Festivals, Culture Festivals.
From the game's encyclopedia
An Activity Phase during a Royal Tour in which a Vassal is expected to entertain their Liege, which often results in an increase in Popular Opinion and Cultural Acceptance.
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.
Cultural Festival is an entry of Crusader Kings III's in-game encyclopedia (Advanced Activities). The game explains it: "An Activity Phase during a Royal Tour in which a Vassal is expected to entertain their Liege, which often results in an increase in Popular Opinion and Cultural Acceptance." It is part of Tour. The game also links it as Culture Festival, Cultural Festivals and Culture Festivals. Patch 1.20.1.2, from game/common/game_concepts/04_ep2_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/04_ep2_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.