What are the first day and the second day?

A day and B day refer to 1 and 2 days of the lunar calendar.

According to heavenly stems and earthly branches, "A, B, C, D, E, Ji, G, Xin, Man and Ghost" are called ten days' achievements.

Each month is divided into three parts, 1- 10 days, 1 1-20 days and 2 1-30 days, with a period of ten days.

The days from 1- 10,1-20 and 2 1-30 are respectively: A day, B day, C day, D day, E day, Ji day, Geng day, Xin day, Ren day and Deck day.

The ancients used heavenly stems and earthly branches to indicate the date, year, month and day, that is, as a numerical serial number. Dry weather is the most common day. In Huangdi Neijing, when a disease is getting better and better, it is marked by heavenly dryness, which is usually considered as a day.

In the ancient calendar, there will be a record of heavenly stems and earthly branches, starting from A, the next day is B, and so on, and then there is A .. In Huangdi Neijing, heavenly stems are combined with five elements, namely, A, B, D, Fire, Wuji, Tu and Bu. We can know when the corresponding five-zang diseases will get better or worse according to the law of five elements producing grams.

Extended data:

Ten dry words: encounter, enlightenment, gentleness, firmness, harmony, Tu Wei, Shang Zhang, heavy light, mystery, sunshine. The twelve earthly branches say: sleepy head, red heart, taking photos, one hand, holding beard, falling flower, Dunyi, Qi Xie, Qitan, and Dayuanxian.

The simplified heavenly stems and earthly branches: "A, B, C, D, E, Ji, G, Xin, Man and Ghost" is called the ten heavenly stems, and "Zi, Ugly, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai" are called the twelve earthly branches.

Ten heavenly stems and twelve earthly branches are matched in turn to form sixty basic units, and they cooperate with each other in a fixed order to form the chronology of the stems and branches. Heavenly stems and earthly branches's invention had a far-reaching impact, and heavenly stems and earthly branches still uses it today, for calendar, technology, calculation, naming and so on.

Baidu Encyclopedia-heavenly stems and earthly branches