There are a ton of threads on the DISboards currently, and all of them mention that the party is scheduled to end May 31st. More than likely that is because Summer Nightastic is starting June 6th, so a new fireworks show, as well as Electric Light Parade is on schedule for the summer.
You will surely see on here or All Ears if the party will be extended past May.
I too am hoping they extend this for September. We will be there from 9/9-9/16 and would love to attend. We missed out last year, but would love to book it for this year. Will have to keep watching to see.