The Autumn Equinox falls on 22 September. Also called Mabon or the late harvest festival, it is celebrated when day and night are of equal length. It is also the date that the sun enters Libra, the scales, so is a time of balance before the descent into the darkness of winter.
It is also a good time to try to bring balance into your own life. You could spend a little time meditating on the good and the bad things that have happened in the year so far, trying to see each side of an argument or weighing up the pros and cons of a situation.
As this is a celebration of the bringing in of the harvest, suitable things to put on an altar at are ripe apples or other autumn fruit you have picked yourself, home-baked bread or corn dollies.
The Autumn Equinox is also a time of high tides, particularly around the full moon, which is one of the reasons I am writing a meditation on the element of water. I will post it on A Bad Witch's Blog in the next few days, after trying it out myself.
Here are some links giving more information about the Autumn Equinox and also offering several different rituals that are appropriate for the festival: