Hello Lisa!
You must be getting so excited that your vacation is only days away!
Crowds at the Magic Kingdom are typically less crowded during the week and on days when the park doesn't have extra magic hours. Looking at the
calendar for this month, it looks like Monday the 17th and Wednesday the 19th would both be good options.
I would suggest Monday the 17th as there are evening extra magic hours at the Magic Kingdom the night prior, and sometimes that can mean fewer crowds. The logic behind that being that if people were at late extra magic hours the previous night, they'll probably be visiting a different park that day.
Honestly though, you are going at a wonderful time of year. Some schools have already started back up for the year and the crowd levels should be pretty manageable overall throughout your entire trip. If you have park hopping on your tickets you can always move to another park if the one you're in seems a bit too crowded.
I hope this helps! Have a wonderful vacation!!