Can we require employees to work weekends?

Q. Our (medical) office is open seven days a week and 11 hours on weekdays. We rotate our staff so all hours are covered, even if each employee works 40 hours a week only. Recently, our regular weekend person left. Management wants to rotate all employees to cover weekend duties.  Some refuse for reasons like: church, family, health, or even saying that when they were hired years ago, they were not told that they had to do weekend duties. Is it legal for us to impose the weekend duties as part of their job to cover the days we are open? – Operations Manager, Florida