Sept 22nd will be extended evening hours for deluxe guests. But the crowd should be over at Magic Kingdom that night, since it is one of the few days that week that MK isn't closed for a party.
Sept 21st has MK closed at night for a party, so the crowd might be at Epcot at night.
I would be more worried about the Sunday crowd at Epcot during Food and Wine. The locals especially like this festival.