41 C.F.R. § 60-50.3

An employer must accommodate to the religious observances and practices of an employee or prospective employee unless the employer demonstrates that it is unable to reasonably accommodate to an employee's or prospective employee's religious observance or practice without undue hardship on the conduct of the employer's business. As part of this obligation, an employer must make reasonable accommodations to the religious observances and practices of an employee or prospective employee who regularly observes Friday evening and Saturday, or some other day of the week, as his Sabbath and/or who observes certain religious holidays during the year and who is conscientiously opposed to performing work or engaging in similar activity on such days, when such accommodations can be made without undue hardship on the conduct of the employer's business. In determining the extent of an employer's obligations under this section, at least the following factors shall be considered: (a) Business necessity, (b) financial costs and expenses, and (c) resulting personnel problems.