As referenced in Article 8, sections 4 and 5, of the USPS-NALC and USPS-APWU national agreements, the December period (during which penalty overtime regulations are not applicable) consists of 4 consecutive service weeks.
This year, the December period begins Pay Period 25-18, Week 2 (December 1, 2018) and ends Pay Period 01-19, Week 1 (December 28, 2018).
(Source: USPS Postal Bulletin)