Federal law requires employers to accommodate the religious practices of employees unless it would be too burdensome to do so. The Equal Employment Opportunity Commission (EEOC) reports the most frequent accommodations: flexible scheduling, voluntary substitutions or swaps, job reassignments and lateral transfers, modification of grooming/dress codes and permitting prayer during nonworking time.
For additional guidance click here: http://www.eeoc.gov/types/religion.html