Event means a special gathering that many people go to for a purpose to have fun, learn, shop, etc. For example, a musical festival or a farmers market.

Eventuality means something or an action that will happen in the future.

Event is any kind of happening. Eventuality is a happening that is possibly going to take place but it feels more uncertain than event. If you're talking about something in the future, both have a very similar meaning but when talking about the past, then an "event" did happen but an "eventuality" probably didn't by now.