5 C.F.R. § 550.171
Authorization of pay for Sunday work
(a) An employee is entitled to pay at his or her rate of basic pay plus premium pay at a rate equal to 25 percent of his or her rate of basic pay for each hour of Sunday work (as defined in § 550.103).
(b) An employee is not entitled to Sunday premium pay while engaged in training, except as provided in § 410.402 of this chapter.