Can my employer work me outside of my agreed availability?

I just became employed 2 weeks ago upon employment I was asked my availability. Which is open except for weekends. My employer just scheduled me to work both Saturday and Sunday two weeks from now. I work in retail and there are many other cashiers who are open to work those days. Can my employer actually do this or can I refuse to show? Will I possibly lose my job for not being able to show up, since I have it in writing that I can not work weekends from before they even hired me? Please give answers not opinions.