Can an employer threaten disciplinary action for a Saturday overtime, when it is not part of a regular 40hr work week?

My manager told us that we will be working overtime the coming work week, but told us that he is making a mandatory Saturday overtime. He threatened all of the employee's that if we do not show, there will be repercussions if people don't reschedule school, appointments, etc, in order to be there. Can he fire people for not showinp up on a Saturday when the work week is a mon-fri 40 hours a week, when there is already overtime scheduled for the regular work week? Please help! - Is this your question? Add additional information