the.com/autumnal equinox

means The moment, occurring around September 22-23 in the Northern Hemisphere, when the sun crosses the celestial equator heading south, making day and night roughly equal in length everywhere on Earth.

from 'Equinox' comes from Latin aequus ('equal') plus nox ('night') — so literally 'equal night,' a word the Romans built the same plain way we'd build it today. 'Autumnal' traces to autumnus, the Latin word for the season itself, origin uncertain — possibly linked to an older Etruscan word, though that's a guess scholars make carefully, not a fact anyone can prove.