When did the first Geng Day start?

A Geng day is 10 day. Tiangan has 10, which are A, B, C, D, E, G, Xin, N and G, so there is a Geng Day every 10 day. Gengli algorithm:

On the ninth day of the winter solstice, the summer solstice is three nights and three volts. It means: on the winter solstice, it is considered as nine, while on the summer solstice, it enters after midnight.

"Three Geng" refers to the meeting for three days, from the third day of "Geng" to the fourth day of *** 10. In ancient China, time was recorded by combining heavenly stems and earthly branches.

Related origin

"The third watch" means that the three words "Geng" are met from the solstice in summer, and the third watch is the first time. "Young Students Qionglin" said: "The first day is the third night in summer. "

Because the number of heavenly stems is 10, there is a Geng Day every 10 day, such as Geng Zi Day, Geng Yin Day and Chen Geng Day. 365 days a year (366 days in leap year) is not an integer multiple of 10. One day this year is Geng Day, and next year is not necessarily Geng Day. Due to the uncertainty of Geng day, the date of accommodation is different every year.