99% of offer letters are written in a way that is not binding on the company (or you for that matter). It's not actually a contract in almost all situations. Think of it as more of a loose term sheet unfortunately. They can fire you and you can quit before you even start, so they can certainly change the terms of your employment.
However, it is a very shitty and uncommon thing for them to do. It could very well be an honest mistake. I would politely point out that the offer letter stated M-F and have them either correct your schedule (if a mistake) or explain to you why they changed it. In the end they can probably do what they want though.
ETA: One caveat - there are some rare situations where you could argue some sort of promissory and/or equitable estoppel. I have seen cases where a written offer was made which induced someone currenty employed to quit their job and start with a new company (and possibly move, incur expenses, etc). In rare situations you could potential get damages if the new company failed to hire you and you couldn't get your old job back. However damages would be very hard to determine since technically they could fire you the first day with or without cause since you are an at-will employee. You would have a better chance under this theory to recover things like moving expenses, etc. that were a direct result of their offer letter. Lost wages would be hard to recover in even the most egregious situations.
This post was edited on 8/6 at 3:48 pm