Last year was the earliest; May 1. Time will tell if it's the same this year.
I'd say the end of April Disney would know when they'll go on sale cause I remember last year they were telling us they'd go on sale May 1. They actually gave us the sale date beforehand.
I have the dates and links to information about MNSSHP from wdwinfo on the first post. Everyone will be happy to answer any questions you have. Come join us!
MommyBoo! is making a fantastic spreadsheet of everyone who is going in October.