Mobilizon

Find, create and organize events

Is freely licensed?	✅
Decentralization	federated
User interface	gui
Target audience	end users system administrators
Platforms	gnu/linux bsd
Community support	chat
Website	https://mobilizon.org
Source	https://nlnet.nl/gallery/repology.html#repology_Repology
NLNet grant pages	https://nlnet.nl/project/Mobilizon https://nlnet.nl/project/Mobilizon-UX

Mobilizon is a federated events and groups tool so organising meetings does not depend on Facebook Events or Meetup. Communities can self-host (Mobilizon docs; examples only—not a canonical list):

Framasoft / French-language instance
Picasoft associative host

Search and import from proprietary calendars keep organising off Facebook Events.