Greetings Jennifer!
Welcome to the Disney Parks Moms Panel! Mickey's Not-So-Scary Halloween Party is my favorite special event and I have some great news--the 2015 dates for the party were just released today! You can find them (and the dates for Mickey's Very Merry Christmas Party)
in this article on the Disney Parks Blog. I am so excited! Now, for your other question, you are able to search the historical data on promotions through the various Walt Disney World fan sites, however, there is never a guarantee that the same offers will be available this year. I would suggest booking your vacation for what time works best for you and then keeping an eye on the
Special Offers page of the Walt Disney World website. If an offer is released that corresponds to your visit, simply call Disney and let them know you would like to take advantage of the deal. (Please be aware that details of your visit such as resort, room preference and length of stay may need to be adjusted to coincide with the promotion details.)
Have a wonderful time planning your practically perfect vacation and if there is anything else we are able to assist you with, please let us know!
Magical Wishes,
Lauren