[autumnal equinox] just the facts, ma’am

The equinoxes are the two days each year, occurring around March 20 and September 23, when the centre of the Sun spends an equal amount of time above and below the horizon at every location on Earth. The word equinox derives from the Latin words aequus (equal) and nox (night). In practice, the day is longer than the night. This is firstly because the Sun is not a single point of light, but appears to be a disc. The real equality of day and night happens a few days towards the winter side of the equinox. More facts here, including 2006 and 2007 dates plus cultural aspects.