Mobilizon
Federated tool for events.
See also
Project website
Comments & critique of Mobilizon on Petites Singularités forum
Comments on already existing (and used) platforms for events that are used by non-Facebook users and the faulty narrative of "getting of Facebook":
... it is not a matter of “getting people off Facebook or Google” but of proposing a different organizational model. We do not position ourselves in relation to these companies: we are elsewhere, in the creation of collectives and resistance.
Thread has also valuable input from developers from radar.squat, Demosphere and Gancio.