Having an calendar in-game would let people who are not on slack actually know what kinds of stuff is happening! (And it looks/is better imho)

There would be a calendar button on the menu and clicking it would bring up a window similar to WoW's calendar

( https://www.blogcdn.com/wow.joystiq.com/media/2010/11/decembercalendarchangesrlt.jpg )

Users would be able to select a date, create an event and other people could sign up for it.

The creator would be able to set a starting time for the event and choose a world which it would be at.

Since this is a global community I think it would be best to use UTC time to coordinate everyone, and if possible, setting your own time-zone in settings and having VRchat convert the times to your local time.

Before the event starts a notification would appear (As shown in picture) so you can hang out with friends and not forget the time. An option to auto-join the event world 5-15-30 minutes before would be handy too!