Why is Tomb-Sweeping Day "fifteen days after the vernal equinox"?

It is related to "weather phenology". The introduction of astronomy in the book Huainanzi in the Western Han Dynasty includes "The wind will clear up on the 15th day after the vernal equinox, when it refers to B"; The record of Qingming in Hundred Questions at the Age is "Everything grows clean and bright at this time, hence the name Qingming". Although the descriptions in these two ancient books are different, they all tell the same truth, that is, "everything presents a clean and clear phenomenon and feeling." Tomb-Sweeping Day is the fifth of the 24 solar terms. At this time, the sun has passed the equator for two weeks (the vernal equinox solar terms hit the equator directly), and the sunshine in the northern hemisphere is gradually lengthened, with long days and short nights. After beginning of spring, after three solar terms: rain, fright and vernal equinox, the cold in the depths of the earth in winter was taken away and dissipated by the warmth brought by the sun. At this time, spring flowers are blooming, thick green covers the earth, the sky is clear, the air is fresh, everything comes into being at the right time and is full of competition. It can really be described as "the wind is clear and the gas is positive" and "thriving".

2. Understand from a humanistic perspective. "Qing": the sky is light and the clouds are light, but it rises again. The sky is clear and not turbid, transparent and bright; God is pure and selfless, blessed by all things. "Ming": be familiar with things, and it will be clear if you follow the trend. The ancients used the word "Qingming" to declare that future generations should be honest and clean, do things clearly, and be "honest and clean".

Sacrifice ancestors in Qingming Festival, communicate with ancestors, and let them judge whether the actions of strangers in the world are honest or not. As the saying goes, people are doing, and the sky is watching. The sky contains ancestors. Ancestors only bless those who are upright and do things clearly. Therefore, people who post notices should always be vigilant, work hard and always be vigilant. The law has always been strict.