Should I ask my maid of honors boss if she could take the day before the wedding off?
Why would you ask the boss of the maid of honor if she can take the day off?
I want to do the final fitting for the dresses that day
You could interfere with your maid of honor's job. You need to mind your own business. It is not your place to ask for the day off for someone else.
F*ck this is my wedding what the hell. I need her to help me that day, so she needs the day off.
I think you're expectations of a maid of honor are a little bit much. She's doing you a favor. It doesn't entitle you to control all of her time.