The conceptual jump from saving a post to joining an event is too big, I think. Saving is by definition passive - the creator of the post doesn't care if you save it or not. Joining in on an event, be it a movie club or an IRC thing, is quite the opposite. Especially if you include a list of participants on the post itself.
1) to make joining events a separate thing, and have a 'join event' button. Clicking this will add the post to your events (visible to you or not. Maybe something for on your bio page as cards under Recent activity?).
2) to make saving a post the same thing as joining an event. When I would go to my saved posts, I could see a list of upcoming events I've 'saved'.
Both 1 and 2 would have the effect that the event pops up on your feed on the day it happens, and if the user specifies a time, the hours before and after that time and date. Also, might it be an idea to give upcoming events a place on the Community page?
Ninja edit: also, can you replace the month with something like 'sept', 'jul' etcetera? Only Americans use the YYYY/DD/MM format.