Principle of the Leap Year Determination Method

The Principle of Setting Leap Years
1. Yuan, Hou, and Symbol Head Days
The three Yuans: Upper Yuan, Middle Yuan, and Lower Yuan are collectively referred to as the three Yuans. The first solar term of each hexagram is the Upper Yuan, the second solar term is the Middle Yuan, and the third solar term is the Lower Yuan.

For example, in the Kan hexagram, the Winter Solstice is the Upper Yuan, Minor Cold is the Middle Yuan, and Major Cold is the Lower Yuan; similarly for other hexagrams. Each solar term lasts for 15 days: the first 5 days are Upper Yuan, the middle 5 days are Middle Yuan, and the last 5 days are Lower Yuan.

Hou: Every 5 days constitutes one Hou; there are 72 Hous in a year.

Symbol Head Day: Also known as God Arrival Day, it is the starting day for changing cycles and must fall on either Jia or Ji of the Heavenly Stems. If Jia or Ji falls on “Zi, Wu, Mao, You,” it corresponds to Upper Yuan; if it falls on “Yin, Shen, Si, Hai,” it corresponds to Middle Yuan; if it falls on “Chen, Xu, Chou, Wei,” it corresponds to Lower Yuan.

Excerpt from Dun Jia Arena