It is coldest in September, and it begins to pick up in September.

In the nine days, March 9 and April 9 are the coldest, and the day when the nine days end begins to get warmer.

Jiujiu, also called Jiujiu in winter, is a folk method to calculate the date of warm flowers in spring and cold days in China. Generally, March and April are the coldest time, which is the coldest time of the year. When you count to nine "nine days" (998 1 day), spring will be deep and warm, and everything will be full of vitality. It's time for spring ploughing.

Because after the winter solstice, the heat absorbed during the day gradually decreases until March 9 and April 9, when the earth stores the least heat, March 9 and April 9 are the coldest days. The weather began to warm up after the nine cold, and the specific date began to warm up on the day when the nine cold ended.

Liang Chaozong Ba wrote in the Chronicle of Jingchu: "The number of winter solstice days and 998 1 day are the end of cold." Count nine cold days, that is, from the solstice of winter, count a "nine" every nine days until the "99" of 81 days, and the weather will warm up.

Calculation method:

The correct algorithm of "counting nine" begins from winter to the first nonunion day in the future. Therefore, the specific date of "September" every year is not fixed, depending on the first nonyl day after the winter solstice. Another view is that the number nine is counted from the date of winter solstice, so the specific time of "nine" is counted from the date of winter solstice every year, and each "nine" is fixed on the ninth day, 18 and the 27th day after winter solstice. The algorithm of "counting nine" from winter to the sun is not very accurate.

The original algorithm of "Counting Nine" is to count from winter to the first nonunion day in the future (referring to the days prefixed with "nonunion" in the ancient "law of observing cadres and supporting the day"). As the saying goes, "Summer solstice is three years old, and winter solstice is nine." Because of this trouble, it was later counted from the winter of solstice.

The above contents refer to Baidu Encyclopedia-Count Nine.