When did the days get shorter?

The date of shortening the day cannot be fixed, usually in the autumn equinox.

The autumnal equinox is the sixteenth solar term among the twenty-four solar terms and the fourth solar term in autumn. After the autumnal equinox, the position of direct sunlight moves south, and the days in the northern hemisphere become shorter, the nights become longer, the temperature difference between day and night increases, and the temperature drops day by day.

On the day of the autumnal equinox, the sun shines almost directly on the equator of the earth, and the length of day and night is equal all over the world. The autumnal equinox means "equally divided" and "equally divided", which means equally dividing autumn except day and night. The autumnal equinox used to be the traditional "Festival of Sacrificing the Moon". Now Mid-Autumn Festival comes from the traditional autumnal equinox "offering sacrifices to the moon". According to textual research, the Mid-Autumn Festival was originally set on the day of the autumnal equinox, but because this day is not always a full moon, the Mid-Autumn Festival was later transferred from the twenty-four solar terms of the autumnal equinox to the fifteenth day of the eighth lunar month.