Hi Elizabeth,
Thank you so much for visiting the Disney Parks Moms Panel with your question.
Mickey's Not-So-Scary Halloween Party does require a separate ticket that you have to pay for. Since you do not celebrate Halloween, I suggest that you do not go to Mickey's Not-So-Scary Halloween Party. Aside from that, if you decide to visit Walt Disney World in mid-October, you will likely see Halloween decorations and merchandise; however, the most active celebration of Halloween is during Mickey's Not-So-Scary Halloween Party.
As to the best time of year to visit Walt Disney World, I think that there are advantages to lots of different seasons. You mentioned that you are considering mid-October, which is one of my favorite times of year to visit Walt Disney World. I like that the weather is cooling off a bit from the intense head of Orlando in the summer. I also like that you can enjoy the
Epcot International Food & Wine Festival, which includes great kiosks for tasting many different cuisines as well as informative seminars and exciting entertainment. Typically because most schools do not have long breaks in October, crowds are lower.
There are advantages to going at all times of year. My family generally now visits Walt Disney World during school breaks for our daughter. While there may be higher crowds at Walt Disney World at these times, we do not have to worry about her making up schoolwork. If you are not planning around a school schedule, I also really like September as a time of year to visit as it is warm, but tends to be less crowded as families often do not travel when their children have just started school again.
I would love to help you plan more Elizabeth; however, I am limited in the space that I have to answer. Please do write back if you have any further questions!