Hi Olivia!
I've lost count on the number of times I've been to Mickey's Not-So-Scary Halloween Party ("MNSSHP"). I absolutely love that event and can't wait for it each year. Plus, it seems like there's always something new being added.
You asked a great question about the dates for this year's MNSSHP. If my memory serves me correctly, the dates are typically announced, and tickets are offered for sale, beginning in either late April or early May. My best advice would be to keep a close eye on the
Disney Parks Blog for the announcement. If you haven't already subscribed to the blog via e-mail or RSS feed, I'd definitely do that because they offer great content each day.
Here's a little tip from your buddy Derek:
Since you'll be traveling with a large group when you come in September, it probably would be a good idea to make your dining reservations now. Since the MNSSHP doesn't start each night until 7:00pm, you could make 6:00pm dinner reservations at Magic Kingdom restaurants for at least a couple of the nights during your visit.
If it turns out that a reservation falls on one of the nights when a MNSSHP is being held, you'll be fueled-up and ready to go once the MNSSHP starts. If not, then you'll at least have a dinner reservation locked on the books while you still have some time to plan. Or, if one of those reservations ends up falling on a night when MNSSHP is not being held, you can always change the reservation if you'd rather not spend that particular night in the Magic Kingdom.
I hope that helps! If you should have any other questions, please feel free to let me know and I'll be happy to help in any way that I can!
Best regards,
Derek