As a general matter, an employer may change the terms and conditions of employment, including hours or days worked, at will. You would only have recourse against this if--1) There is an actual employment contract setting forth days worked; if so, it can be enforced.2) If you gave up another job or relocated to take this job, and you did so specifically because the employer had represented to you that you would have weekends off--in that case, the fact that you took action to your detriment in reasonable reliance on the representation may be enough to make that promise enforceable.