I went last Sunday April 6th. It rained the whole time, but it was still definitely worth the money. We walked onto every ride, and the fireworks were incredible. Though the party doesn't officially start until 7:00, they let you into the park at 4:00 with your ticket, and it's open until midnight. I'd sleep in that morning, hang out by the pool or do some shopping, then be at MK at 4:00 p.m. ready to go.
To find the dates, go to the WDW section of Disney.com and search for Pirate Princess Party. There's a page just for it and you can find the dates, but right now the last scheduled date is June 4th.
I would definitely go again.