My employer informed the employees on Friday that all employees must work the next day even though it is not a regularly scheduled work day. This will bring all the employees into overtime. The ones that had prior appointments made for the Saturday was told they must be present at work or they will face disciplinary action. Is this legal?

Unless the employees in question have a bona fide contract that limits the employer's rights to require overtime, yes, this is legal.

Thank you very much.