http://www.wdwmagic.com/events/mickeys-not-so-scary-halloween-party/news.htm
If you scroll through the posts, this lists the dates that sold out last year, and when they sold out.
September 27 (sold out day of; I believe this was the last date that CMs were able to get discounted tickets)
October 10 (sold out October 9)
October 11 (sold out day of)
October 14 (sold out day of)
October 17 (sold out October 15; this party fell during an Orlando school break)
October 29 (sold out October 28)
October 31 (sold out October 4)
Nov 1 (sold out October 31)
Both Columbus Day and Canadian Thanksgiving were on October 14th last year, which I think accounts for all the sellouts around that weekend, and the pattern will likely repeat this year. The Thursday party that falls during Orlando's fall break has sold out the last two years, so I'd expect it to sell out again this year. That Thursday party is discounted while the party on the next day (Friday) isn't, so the local crowd seems to prefer the discounted party.
If I had to bet, I'd put my money on September 26th, as well as the 9th, 10th, 13th, 16th, 30th, and 31st of October selling out.