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Equinox
Equal Day/Night
Equinox means 'equal night' in Latin, referring to the time when daylight and night are nearly the same duration. 
Happens Twice
Occurs two times each year, so specify by saying 'vernal' for spring, or 'autumnal' for fall. 
Cultural Significance
Many cultures celebrate equinoxes, associating them with renewal or thanksgiving rituals. 