Susan,
Typically the tickets for
Mickey's Not-So-Scary Halloween Party go on sale in early May. The dates will be announced not too long before that.
Now nobody but Disney knows the actual dates, but you can assume that the dates will run from mid-September through the first days of November. In September they only seem to be held on Tuesdays and Fridays. Then when you get into October the schedule accelerates to 3-4 nights a week. The key pattern there is that they don't seem to hold it on Saturdays or Mondays.
But for your trip you can expect to hear dates in the next several weeks, and for now can assume that options will largely fall on Tuesday and Friday nights.
As for times, the party usually runs from 7 pm to midnight. Because it is an extra cost, you really want to be sure to take full advantage of the party hours. Don't plan a big, sit-down meal in the middle of party hours. You can do that some other evening!
Write me back if you have any follow-up questions!
Have a magical vacation!