Hi Kate!
When it comes down to it, there's not really a particular day or days that Magic Kingdom Park is busier or less busy than others. Lots of factors come into play when determining crowd levels in any of Walt Disney World Resort's four theme parks: Holidays, Special Events, Extra Magic Hours, etc.
From my own personal experience, I tend to believe that Sundays are the least busy, because guests are either leaving at the end of a week's vacation, or arriving on that day to begin visiting the theme parks on Monday. Mondays, in turn, always feel busier because guests that arrive on Sunday tend to begin their vacation the next morning by visiting Cinderella Castle in Magic Kingdom. (I know that I don't truly feel my Walt Disney World Resort experience has begun until I gaze upon the castle!) That being said, I've visited Magic Kingdom on Mondays when I've felt like crowd levels were close to perfect.
When my family is staying at a Disney resort, we take advantage of
Extra Magic Hours, and choose which parks to visit on which days depending on the EMH schedule. During Extra Magic Hours, guests of select Walt Disney World Resort hotels get to enjoy spending extra time in the parks before opening and after closing! It's amazing how much we can see and do during those hours, so we make the most of them!
It's also important to make
Advance Dining Reservations for meals and use
FastPass+ to reserve ride times for your favorite attractions. By planning a bit ahead of time, you can breeze through your day at Magic Kingdom even when it may be crowded.
I hope this helps answer your question! Have a magical day! - Diane W.