Almost always read zhèng. The single exception is
正月 (first lunar month, and the derived
正月十五 for Lantern Festival), where it shifts to first-tone zhēng. Tradition says this avoidance dates to the Qin emperor Ying Zheng (
嬴政) — the calendar month was renamed to dodge his name's reading. Outside the lunar-calendar context, zhēng never appears.