If you require employees to work Fridays, Saturdays or Sundays, be aware that some employees may object on religious grounds. If they do, you're required by Title VII to make reasonable accommodations for sincerely held religious beliefs. And, surprisingly, that right may extend to the entire day off, not just long enough to attend religious services.
You can deny an accommodation only if it creates an "undue hardship" on the organization.
Recent case: Bradley Baker worked full time for Home Depot for several months before joining a new church. The church stressed Sunday service attendance and the importance of Sunday as a day of rest. After initially meeting this Sunday-off request, Baker's supervisor relented, offering to shift him to part-time status or allowing only Sunday mornings off.
No good, said the court. Cutting his hours to part time would end his eligibility for medical insurance and, therefore, was a benefit cut. And the court didn't believe that giving Baker the entire day off would cause an undue hardship on the retailer. (Baker v. Home Depot, No. 05-1069, 2nd Cir., 2006)