When is the Autumn Equinox and how is it celebrated?

The Autumn Equinox is celebrated on the 22nd/23rd September in the northern hemisphere, and the 20th/21st of March in the Southern Hemisphere. At this time the Sun is directly overhead at the equator and day and night are of approximately equal lengths. At equinox, the Sun rises everywhere on Earth (except the Poles) at about 6am in the morning and sets at about 6pm in the evening (though this can vary a little when time zones and daylight saving etc are taken into account).
NB: At the North Pole the Autumn Equinox is effectively the Polar sunset whilst the South Pole experiences it as a Polar sunrise.
Also known as the September equinox, the event signifies the first day of autumn in the UK. The autumn equinox is also used to determine the date of the traditional harvest festival which is held on the first Sunday after the first full moon following the autumn equinox.

The 2010 Autumnal Equinox takes place on September 23th, at 4.03am UK time