What does "June 21st in the first half of the year and August 23rd in the second half of the year" mean in Twenty-four solar term song?

The first half of the year is June 2 1 day, and the second half is August 23, that is to say, June to June is the first half, and July to June is the second half.

Twenty-four solar terms reflect the correct understanding and understanding of the time conversion law of the annual movement of the sun in ancient China. Spring fills the earth with hope, and everything presents a scene of prosperity. After long-term observation of the sun, the ancients realized that the changes of cold and summer are not only consistent with the changes of sun shadows, but also have laws to follow. Using the principle of immediate effect, the ancients in China gradually realized the annual change of the sun.

They gradually mastered the four most important time nodes in a year: winter solstice, summer solstice and vernal equinox, and autumn equinox (two minutes and two minutes). For this reason, in Oracle Bone Inscriptions, most time-related words have the word "day". In Yao Dian of Shangshu, the vernal equinox is called the middle of the day, the autumnal equinox is called the night, the summer solstice is called the eternal sun, and the winter solstice is called the short day. The ancients relied on the scientific observation of the sun, accumulated a little, and finally formed a perfect and systematic knowledge of the time of the twenty-four solar terms.