Yep. This hour will most likely be the final confirmed hours unless the private event is canceled. Parks close early before 6 PM besides Animal Kingdom, (it can close as early as 5 PM), are usually due to Private Events, Bad Weather like hurricane which all parks close at 5 PM on October 6, and all parks close without not even a single minute open on October 7. It can also be closed due to events. It is normal for parks to close before Midnight even on weekends except Thanksgiving week on November. This happened to
Disneyland which closed at 5 PM once on the day before my bday which is on September. The hours will start to get short in August for Disneyland and get longer in May. However, here the hours is significantly longer throughout the year in which the Hours get shorter on October for Hollywood Studios opening from 9 AM to 7 PM (IDK why on this one) and now Animal Kingdom 9 AM to 7 PM (new) (possibly due to the weather starting to get colder and sunset being earlier). Epcot which opens from 9 AM to 9 PM usually, doesn't effectively change at all because it always close at 9 PM (especially in June and July in which they should close at 10 PM or 11 PM). Magic Kingdom are significantly longer in May to October opening 8 or 9 AM to 10 to 1 AM (except some nights in September in which it closes at 9 PM on some nights and for September and October, 7 PM for the event) In November it will always open 9 AM to 9 PM almost every day and continues to be like that until busy days. Also from Late October to March, one of the Waterparks will start to close for rehab. It opens usually from 10 AM to 5 PM with the exceptions of June and July. After Halloween, pretty much every park opens after 8 AM and will always open at 9 AM until Summer rolls back.
If you are planning your visit and want to avoid crowds: Visit in November (except Veterans Day and Thanksgiving Week), December (except Christmas time and New years), January (except MLK Day), February (Don't know certain days exception but probably Valentine's Day), and the First Half of March (Spring Break, the park hours starts to get longer), and April (except days when Magic Kingdom still opens after 9 PM). September might be good to fit in this, but it is left out due to Halloween Party events and most hours that aren't Halloween occur are very long.
If you want to Visit on Long Hours: Visit in May (some days), June, July, August (hours sometime starts to get bit shorter except select days), September (except select days and Halloween Party events unless you have a Party Ticket Event where you can stay until Midnight), October (except on Halloween Event days unless you have a Party Ticket), and all of the exceptions on the best time to visit.
Also January 23, 2017 is scheduled to have Magic Kingdom close at 6 PM, suggest you buy a ticket to go to other parks on that day. Though the hours can change.
Well for Disneyland, November is usually the opposite of this. This year for June it looked like this: for Disneyland it opens at 8 AM to 11 PM (Saturdays 8 AM to 12 AM) and California Adventure 8 AM to 9 PM. July opens 8 AM to 12 AM and 8 AM to 10 PM. August starts to reduce as it opens 9 AM to 12 AM and 9 AM to 10 PM. At the end will reach open 10 AM to 9 PM and 10 AM to 8 PM. Septembers continues with the 10 AM to 9 PM pattern and Labor Day it opens 8 AM to 12 PM and 8 AM to 10 PM, this happens on some weekends. Though some days can open at 9 AM even on weekdays. On Mondays and Wednesdays of last week of September, it opens 8 AM to 6 PM and Fridays follows 8 AM to 7 PM with California Adventure closing at Midnight. Almost Every month will have a day that open at 10 AM and closes at 9 PM or 8 PM.