I think it should continue to start the last weekend of September. If you're pulling out all of the stops for the 20th Anniversary, why lessen the nights? I'd add more if it was up to me!
And the event is kind of called Halloween Horror Nights. It'd be a bit silly to leave out Halloween night, no? Although, rather than using the last weekend of September, you could extend it a week past Halloween. That would eliminate the problem with ending on a Sunday if Halloween Night was added, even if going past the night by almost an entire week could seem odd.
Just my two cents though.