In the Northern Hemisphere, the equinox is celebrated as Mabon, also called Harvest Home. Astronomically, the event falls on the 22nd. The precise date and time can be found at archaeoastronomy.com.
Mike Nichols says of the day:
Mythically, this is the day of the year when the God of Light is defeated by his twin and alter ego, the God of Darkness. It is the time of the year when night conquers day.
The metaphor for the natural solar cycle is perfectly clear, and easily appreciable by naturalists. Likewise with the agricultural myth of John Barleycorn, personification of the ripened grain:
Often this corn spirit was believed to reside most especially in the last sheaf or shock harvested, which was dressed in fine clothes, or woven into a wicker-like man-shaped form. This effigy was then cut and carried from the field, and usually burned, amidst much rejoicing. (Nichols)
Jon Cleland Host describes how his family observes Mabon:
Fall harvest décor is appropriate, with dried cornstalks, squash, gourds, Indian corn, etc. The social aspect of our lives is highlighted by this harvest theme – a time when friends and family get together for the harvest. …
Dinner will of course have a harvest theme, including squash, homemade bread, cranberry sauce, etc. … One part of the ritual is often the pouring of a little wine at the base of the trees in our yard as thanks for the summer shade and the coming fall colors. (Naturalistic Paganism yahoo group)
Another great activity, courtesy of Jonathan Blake of the Naturalistic Paganism yahoo list, is measuring your latitude on the equinox using only the sun, a stick, and some basic calculations.
Glenys Livingstone of Pagaian, celebrates this as a time of abundance and thanksgiving, but also of loss. She associates it with the myth of Persephone’s descent into the underworld, ritually enacting a moment of “letting go”:
”(name), I give you the wheat – the Mystery – the knowledge of life and death. I let you go as Child – Daughter/Mabon65), most loved of Mine … you descend to Wisdom, to Sovereignty66. You will return as Mother67, co-Creator with me. You are the Seed in the Fruit, becoming the Fruit in the Seed. Inner Wisdom guides your path.”
Response: “It is so. I am Daughter (Child/Mabon/Young One), becoming Mother – Seed becoming Fruit. I am deepening into/descend to, Wisdom, into Sovereignty. The Mother knowledge grows within me. (vary and add words as desired to express self)
Those in the Southern Hemisphere celebrate Eostar at this time.