Swallows fly back from the south. When is spring?

According to the twenty-four solar terms before and after the vernal equinox, swallows return at the vernal equinox, and the shore is willow green, which is divided equally day and night. In ancient China, the vernal equinox was divided into three periods, one was the arrival of mysterious birds, the other was thunder, and the third was electricity. Swallows fly back from the south after the vernal equinox. When it rains, the sky will thunder and lightning.