Why do the dates of vernal equinox, summer solstice and autumn solstice change?

In fact, all 24 are on the same day (Gregorian calendar) or one or two days nearby. Because solar terms are determined by the ecliptic and based on the solar calendar, the current Gregorian calendar is also the Gregorian calendar, so it is basically the same as the Gregorian calendar, with a difference of at most one or two days.

In addition, solar terms have nothing to do with the lunar calendar (the lunar calendar in the lunar calendar is actually a mixture of yin and yang, and the yang part is the solar terms). The lunar calendar is based on the moon, so the days of the lunar solar terms are different.

Gregorian calendar is a pure solar calendar, and the date is completely determined by the position of the earth around the sun. The same is true of the 24 solar terms. Therefore, the date of the 24 solar terms in the Gregorian calendar is basically fixed and will not float for more than one day.

The lunar calendar is a combination of yin and yang, and its date is mainly determined by the position of the moon around the earth. The position of the earth around the sun is only used to add leap months to keep the months roughly in sync with the seasons. Because its date is not determined by the earth's revolution around the sun, it has no fixed correspondence with the 24 solar terms.