59 Dano
I just want to make friends!
How can they change your day off when you signed an offer letter that said you have Saturday and Sunday off?
An offer letter is only as strong as they want it to be but I've never seen one that said a schedule would never change. Every one I've ever seen has included language to the effect of "your schedule is subject to change to meet the needs of the company." You'd still have the opportunity to bump a lower-seniority employee (in most cases, at least) and retain your M-friend status. Your other option is to take the 90 days to find another position.