September 23

Astronomical significance

The equinox — when the Sun crosses the celestial equator — falls most often on September 22 or 23. On the equinox, day and night are close to equal length worldwide, though exact equality depends on atmospheric refraction and how sunrise and sunset are defined. Because of leap-year rules and the way the calendar approximates the tropical year, the precise timing shifts slightly from year to year.

Calendar, seasons and astrology

September 23 sits near the boundary between summer and autumn in the Northern Hemisphere and between winter and spring in the Southern Hemisphere. In popular astrology the Sun typically enters the sign of Libra around this date, though the boundary varies by year and by the astrological system used. Historical calendar reforms, most notably the adoption of the Gregorian calendar, affect how fixed dates align with the seasons over long periods.

Observances and cultural notes

• Bi Visibility Day (also called Celebrate Bisexuality Day) is observed on September 23 in many countries to raise awareness and visibility of bisexual people.
• International Day of Sign Languages is observed on September 23, recognized by the United Nations to promote the linguistic identity and rights of deaf people who use sign language.
• Saudi National Day is celebrated annually on September 23 to mark the founding of the modern Saudi state in the early 20th century.

Notable facts and distinctions

The date’s relationship to the equinox makes it a recurring marker in seasonal calendars and ritual cycles in various cultures. Because the same ordinal day can be numbered differently in leap years, references to September 23 sometimes mention leap years when explaining its place in the annual sequence. Over centuries, small calendar adjustments and timekeeping conventions have caused occasional variation in which day the equinox and related events fall.