What’s the policy on this?

I’ve been working here a year and a half. When I applied I put I could work Friday’s and Saturdays. About a year ago I stopped working Saturdays and took it off my schedule. Today my manager said I had to open up 2 of the 3 weekend days because I had them open when I applied.. what’s the policy on this? I can’t work weekends right now as those are the days I’m finishing up my education.