Welcome back, Natalie!
You are a Disney planner after my own heart! Planning early is always the way to go to make sure that everything is all set when you arrive at Walt Disney World!
Mickey's Not So Scary Halloween party dates are released late Spring. The 2017 party dates were released in May. So, I would think it will be about that time for 2018. Once the dates are released, tickets go on sale sometime soon after that.
I would suggest to go ahead and make dining reservation once the dates open for 2018, and if you need to change those dining plans after the party dates are released, you can do that. Getting those advanced dining reservations are sometimes difficult to secure. So if there is a restaurant high on your list, then you can plan your party time around that as the party happens multiple times per week.
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Boo to you, Natalie! Keep the questions coming as you have more! We're delighted to assist you!
Korliss