What do you mean the vernal equinox comes and goes?

China's earliest calendar, which combined astronomy, meteorology and phenology to guide agricultural activities, divided a year into 24 solar terms, and each solar term was divided into three periods (the beginning, the middle and the third), with 72 periods in a year, each of which was accompanied by a symbolic natural phenomenon.

The first time after the vernal equinox is when birds arrive. Yuan bird is a blackbird, jiyan word.

The vernal equinox comes and goes, talking about phenology, saying that swallows fly at vernal equinox and at autumnal equinox.

China has a vast territory, different climate and different phenological phenomena. The seventy-two periods in ancient times were formulated according to the Central Plains (roughly along the Yellow River Basin) and were basically applicable to the Central Plains.