When the hours for September are posted and MK is listed as closing at 7pm, you can be sure that is a MNSSHP night even if the dates have not been "officially" announced.
If you make an evening ADR anytime up until 7 pm, Disney won't cancel it just because it is a party night. However you will be ushered out of the park when you are finished eating if you don't have a party wristband. If you book an ADR past 7 pm, you will need a party ticket or will have to change your ADR once the official dates are announced. Disney is not going to cancel any of your ADRs, that is your responsibility. If for some reason a restaurant you booked is not serving, Disney will contact you.
And FYI, Sundays are usually party nights.