What day did the ancient Chinese refer to as New Year's Day

Ancient China's New Year's Day refers to the first day of the first month of the year, the word New Year's Day means the initial day, that is, the beginning of the year. However, although the meaning of the word new year's day has not changed, but in each dynasty the day is not the same, for example, the Shang dynasty is the first day of December, the Qin dynasty is the first day of October, until the Han Emperor Wu Di when the official unification for the first day of the first month.