PART-TIMERS OWED BACKPAY
On August 23, 2011, OPM issued final regulations implementing a court decision requiring the payment of Sunday premium pay to part-time employees, as well as to full-time employees. The decision was Fathauer v. United States.
If your bargaining unit has any part-timers who might have worked on a Sunday in the last few years, they may be entitled to back pay. Check with your union attorney or other experts. See also this OPM regulation and this OPM memo.