The time of accommodation is not fixed every year, and the length of accommodation is also different. You need to check the almanac to calculate it. Simply put, we can use the formula of "Xia San Geng" to express the lodging date, that is, from the third Geng day in Xia Hou, the initial lodging is 10 day, the intermediate lodging is 10 day or 20 days, and the final lodging is 10 day. In ancient China, the "trunk-branch calendar" was popular, and 60 groups of different names were used to record dates with 10 heavenly stems and 12 earthly branches, which were recycled. The day with the word "Geng" is called "Geng Day". The word "Geng" in Geng Day is the seventh word in "A, B, C, D, E, Ji, G, Xin, Ren, Gui" 10 day, and it is repeated every 10 day. Starting from the summer solstice, according to the date of trunk, the third Geng day is the first, the fourth Geng day is the middle, and the 1 th Geng day after beginning of autumn is the last. When there are four Geng days between the summer solstice and the beginning of autumn, the middle part is 10 days, and when there are five Geng days, the middle part is 20 days. It seems that the emergence of Geng Day will affect the length of abdomen in the morning and evening. So some years will be 30 days and some years will be 40 days.