Depends on what your biggest priority is - lowest crowd or most entertainment running. Thurs/Fri/Sat is the best bet for full entertainment, as even if Fri they are doing the halloween party, sat they will also have evening shows. Sun/Mon/Tues may be slightly lower crowds, but park hours can be shorter and bigger chance of fewer evening shows available.
If you need to minimize airfare though, flying on sat/wed is probably your best bet.
We did Disney in September 2016, and did a Wed/Thur/Fri/Sat. It was great, but crowds were significantly bigger Friday and Saturday, especially Friday afternoon and evening (halloween party was going, but we did not attend and went to DCA which was SUPER crowded as everyone else like us went over there too!).
Hope that info helps! It is likely that crowds will be high, but also that they will keep hours more extended than in previous years and have a full slate of entertainment available all week to help offset the number of people.