Hi Jennifer,
I absolutely love Mickey's Not-So-Scary Halloween Party, so that'll be such an awesome way for you all to celebrate your son's birthday!
Great question about the schedule. I think in years past, the schedule for Mickey's Not-So-Scary Halloween Party was released in the early May timeframe. So, unfortunately we all will have to wait a little while before we can make plans for next year's festivities!
I have a pause about advising you to just look at this year's schedule to try to figure out the exact days they'll likely schedule next year's party because things tend to change each year. However, from a high-level planning perspective, I seem to recall that the most recent years' dates began sometime in the second week of September.
Here's a little tip from your buddy Derek:
The Disney Parks Blog is without a doubt the best (and most reliable) source for breaking news about Walt Disney World. Plus, they're the first to release official schedules for upcoming events like Mickey's Not-So-Scary Halloween Party.
So, if you haven't subscribed to the Disney Parks Blog yet, then I highly recommend it. Or, you can also "Like" the Disney Parks Blog page on Facebook and you'll get the same content delivered to your news feed each day.
(Because I'm such a Walt Disney World fanatic, I do both so I can make sure I don't miss anything!)
I hope that helps! Please feel free to write back if you should have any additional questions and I'll be happy to help in any way that I can.
Best regards,
Derek