From the National Association of Letter Carriers:

As referenced in Article 8, Sections 4 and 5, of the National Agreement between the NALC and the U.S. Postal Service, the December period during which penalty overtime regulations are not applicable consists of four consecutive service weeks.

This year, the December period begins Pay Period 26-15, Week 2 (Dec. 5, 2015) and ends Pay Period 01-16, Week 1 (Jan. 1, 2016).