Emperor Wu introduced the Taichu calendar in 104 BC, establishing the first month as the beginning of the Chinese year.
The reform took place during the reign of Liu Che, the Han emperor commonly called Emperor Wu of Han. The Taichu, or “Grand Inception,” calendar replaced the earlier Qin-era Zhuanxu calendar, which had treated the tenth month as the start of the official year. Taichu restored the first lunar month as New Year’s month.
The reform was part of a broader effort to improve calendrical and astronomical calculations. Court officials, including the historian Sima Qian and the astronomer Deng Ping, helped develop the system. It also incorporated the framework of 24 solar terms, which linked calendar reckoning to the Sun’s position and seasonal change.
“Emperor Wu” is the accepted answer in this Han-dynasty context; the fuller historical name is Emperor Wu of Han. Emperor Guangwu, Emperor Wen, and Emperor Gaozu were also Han rulers, but they were not the emperor associated with the Taichu calendar reform.