Today (September 22, 2012) is the autumnal equinox, a significant date on the religious calendars of Wiccans and other Neopagan groups.

An equinox occurs when the Sun is precisely aligned with the plane of the Earth’s equator, such that our planet’s rotational axis is not tilted toward or away from the Sun; the incoming solar rays are, for a brief time, directly perpendicular to the equator.

On such an occasion, day and night on Earth are of roughly equal length; around the entire globe, neither the day nor the night is significantly longer or shorter than the other. (The term equinox is from the Latin aequus nox, “equal night.”)

There are two such equinoxes each year. In Earth’s Northern Hemisphere, the vernal equinox falls on or about March 20, while the autumnal equinox is on or about September 22. The vernal or “spring” equinox traditionally marks the beginning of the spring season, while the autumnal or “fall” equinox marks the beginning of the fall or autumn season.

(In the Southern Hemisphere, where the prevailing seasons are the opposite of those current in the north, things are the other way around. There, the spring or vernal equinox is in September, while the fall or autumnal equinox is in March.)

In many cultures worldwide, traditional spring (planting) and autumn (harvest) festivals have long been associated with the dates of these equinoxes. Many contemporary Neopagan religions, such as Wicca, celebrate holidays known as Mabon on the autumnal equinox, and Ostara on the spring equinox.

As I write, therefore, Wiccans and other Neopagans in the Northern Hemisphere are today celebrating the autumnal or fall equinox holiday of Mabon, while their fellows in the Southern Hemisphere are simultaneously celebrating the vernal or spring equinox holiday of Ostara.

Mabon and Ostara are two of the eight major annual festivals (known as Sabbats) that comprise the Wiccan religious holiday calendar. The natural cycle of the seasons is known as “the Wheel of the Year,” and Wicca’s eight Sabbats are spaced evenly around the perimeter of that wheel. Mabon and Ostara are directly opposite each other on this seasonal wheel, which is why when one festival is being celebrated in one hemisphere, its polar-opposite counterpart is being celebrated in the other hemisphere.

Of these eight annual Sabbat festivals, four are classed as “lesser,” and four as “greater.” The two equinox festivals of Mabon and Ostara, together with the two solstice festivals (Midwinter or Yule at the winter solstice, Midsummer or Litha at the summer solstice) comprise the lesser Sabbats. The Wheel of the Year is completed by the four greater Sabbat festivals of Samhain (Summer’s End), Candlemas or Imbolc (Brigid’s Day), Beltane (May Day), and Lammas or Lughnasadh (First Harvest).

Mabon, celebrated on the autumnal equinox (which, in the Northern Hemisphere, is today), is essentially a festival of traditional thanksgiving for the fall harvest, which makes survival possible during the long winter months ahead.

Ostara, observed on the vernal or spring equinox (which, in the Southern Hemisphere, is today), is a celebration of the ending of those long cold winter months, honoring the birth and renewal of spring, and hailing the return of fertility to the Earth.