It depends on how difficult it would be for you to do so. You are legally required to work with your employees to make it possible for them to practice their religion. This might include not scheduling an employee to work on an important religious holiday. However, you are not required to offer this accommodation if it would cause a hardship on your business or other workers (for example, if it would upset your seniority system).
For information on religious discrimination and accommodations, see Nolo's article Religious Discrimination in the Workplace.
For a practical guide to workplace harassment and discrimination for managers and HR professionals, get The Essential Guide to Handling Workplace Harassment & Discrimination, by Deborah C. England (Nolo).