When are the spring equinox, summer solstice, autumnal equinox and winter solstice?

Vernal Equinox: The fourth solar term of the twenty-four solar terms, on March 20 or 21; it means the middle of spring, when day and night are of equal length. Summer Solstice: The tenth of the twenty-four solar terms, it falls on June 21st or 22nd; it means that the heat is coming, with the longest day and the shortest night. Autumn Equinox: The sixteenth solar term of the twenty-four solar terms, on September 23 or 24; it means the middle of autumn, when day and night are of equal length. Winter Solstice: The 22nd of the 24 solar terms, on December 21st or 22nd; it indicates the arrival of cold winter, with the shortest day and the longest night.

Let’s talk about “Spring Equinox and Autumnal Equinox” first. Literally, it can be interpreted as "equally divided day and night" in Chinese characters. The vernal equinox is the day in spring when day and night are equal. The autumnal equinox is the day in autumn when day and night are equal.

Let’s talk about “winter solstice and summer solstice”. Literally it can be interpreted as "winter nights are coming to an end" in Chinese characters. The summer solstice can be interpreted as "the end of summer days". Some people interpret "to" as "arrive", which is a misunderstanding. The winter solstice definitely does not mean that winter has arrived, and the summer solstice definitely does not mean that summer has arrived. If you have heard someone say this before and believed it to be true, you should correct it quickly to avoid being treated as a joke.

"Zhi" here means "extremely", which means "the end". What is the end? Winter means "the nights are over", and summer means the days are over. Or to put it another way: the winter solstice is when the shadow length at noon ends, and the summer solstice is when the shadow length at noon ends. In astronomical terms, the winter solstice is when the sun reaches the Tropic of Capricorn and it is time to turn back to the north. The summer solstice is when the sun reaches the Tropic of Cancer, and it’s time to turn back and head south.