It's fun to pick apart the opening times of the Disney parks when a new month is released to the public. Very recently, Disney World's August 2020 calendar was shared, and it appears - going off the opening/closing times - that Mickey's Not-So-Scary Halloween party is going to be starting even earlier this year.
The calendar indicates that the first night of the 2020 Mickey's Not-So-Scary Halloween party will be on August 13th.
No official announcement has yet been made, and tickets are not yet on sale. Although tickets were released in January last year for the 2019 party dates.