The Autumnal Equinox Day marks the moment when day and night are approximately equal in length, occurring around September 22 or 23 in the Northern Hemisphere. This event signals the official start of autumn, as the sun crosses the celestial equator moving southward. Many cultures celebrate this day with various traditions and festivals, recognizing the change in seasons. The equinox is also significant in astronomy and agriculture, as it indicates the time for harvesting crops and preparing for the colder months ahead.