Mabon or the Autumn Equinox is usually celebrated around September 21st, though it can occur as early as the 20th or as late as the 23rd, depending on the timing of the actual Astrological event. Mabon occurs on the date which the sun enters the sign of Libra, the scales. Night and Day are equally balanced at Autumn Equinox but soon that all changes and we'll have longer nights and shorter days. The days will continue to grow shorter until the Wheel of the Year finds its way to Yule.
Mabon brings decomposition that reaches fullness at Samhain.
Mabon is the primary harvest festival and witches pay homage to retreating daylight, and prepare for the coming winter. This holiday symbolizes the God in old age and readying for his death and rebirth.
Even though this holiday is a bit more somber than the others, it is one where we give our thanks for what we have received throughout the past year. This is also a time for witches and pagans to give back to their communities, and generally share their harvests.