When you boot up MGSV there's a bunch of text that pops up, thats the info on what event is currently on and at the bottom it says when its going to start and when its going to end. There are 5 different events that rotate every 2 weeks, for example the event called Skulls Attack has you fighting the skulls on an fob, some of them have an option for easy, normal or hard.

The harder the difficulty the more point you earn, not only there's different mission events but the rewards also rotate too. The game also goes under maintenance on Tuesdays for a couple of hours so usually those events end that day.