The first-tone reading zhēng survives only in calendar-month words. The standard explanation: when the Qin First Emperor Ying Zheng (
嬴政) unified the writing system, his personal name was read zhèng, so the same character in the month-name
正月 was tone-shifted to zhēng to avoid the taboo. The pronunciation stuck for two thousand years even as the name-avoidance custom faded.