5 C.F.R. § 550.1001
This subpart implements 5 U.S.C. 5550a , which permits an employee whose personal religious beliefs require the abstention from work during certain periods of time to elect to engage in overtime work and earn a special form of compensatory time off to make up for the time lost in meeting those personal religious requirements. Religious compensatory time off differs from other forms of compensatory time off in that the sole purpose is to adjust an employee's work schedule to accommodate a religious observance. The employee earns religious compensatory time off by spending an equal amount of time in overtime work before and/or after taking time from the employee's scheduled tour of duty to meet personal religious requirements. Hours worked to earn religious compensatory time off provide a time off credit in lieu of any pay that would otherwise be payable for that work.
5 C.F.R. §550.1001