Do you have legal grounds to fire them?

You have an employee that you clearly informed when you hired that they must work on Saturdays and Sundays especially during the busy season - this was a condition of employment.

The employee agreed to this and you did not anticipate any problem. They have just told you that they refuse to work on Sunday's as their church forbids working on a holy day.

Your first thought is to fire them immediately, but concerned that you may be sued. in canada bc