The Day Master is a specialized term used in Chinese Four Pillars of Destiny (Ba Zi). In Ba Zi, each of the four pillars—year, month, day, and hour—consists of two characters (hence the name "Eight Characters"). Each pillar has one Heavenly Stem (tiangan) and one Earthly Branch (dizhi), and the Heavenly Stem of the Day Pillar is the Day Master.

The Day Master represents oneself. It is the center of the entire Ba Zi chart, and the Ten Gods such as Zheng Guan (正官), Qi Sha (七杀), and Zheng Yin (正印) are all relative to the Day Master (for example, if Ji (己) is the Day Master, the Yang Heavenly Stem Jia (甲) Wood overcomes the Yin Heavenly Stem Ji (己) Earth, thus Jia (甲) is the Zheng Guan. Similarly, Ding (丁) Fire is the Pian Yin).