Why isn't the first day of spring on the vernal equinox?

Historically spring starts on the day of the vernal equinox, which usually occurs on the night of 20/21 March
i take it for some reason you think the equinox starts early maybe you have been miss informed.

The Vernal Equinox is not actually when day and night are the same length, as the word “equinox” would suggest.

Instead, the Vernal Equinox is the day when the sun crosses over the celestial equator.

all you have to do is read my response it explains it all

March 20, 2009 is both the date of the vernal equinox as well as the first day of spring.