At my job I am paid full commission and my manager says that is absolutely mandatory that we work one Saturday out of the month however, that would make me work over 40 hours in a work week. Can they do that?

Yes, they can.

There is nothing magic about the 40 hour point except that this is where the government chose to draw the line in the sand with regards to overtime. Nothing whatsoever under Federal law or the law of any state restricts an employer's right to have an employee work more than 40 hours a week, no matter how that employee is paid.