"Technically, an equinox is an astronomical point and, due to the fact that the earth wobbles on its axis slightly , the date may vary by a few days depending on the year. The autumnal equinox occurs when the sun crosses the equator on its apparent journey southward, and we experience a day and a night that are of equal duration. Up until Mabon, the hours of daylight have been greater than the hours from dusk to dawn. But from now on, the reverse holds true.
Mabon marks the middle of harvest, it is a time of equal day and equal night, and for the moment nature is in balance. It is a time to reap what you have sown, of giving thanks for the harvest and the bounty the Earth provides. For finishing up old projects and plans and planting the seeds for new enterprises or a change in lifestyle. Mabon is a time of celebration and balance.
This is the time to look back not just on the past year, but also your life, and to plan for the future. In the rhythm of the year, Mabon is a time of rest and celebration, after the hard work of gathering the crops. Warm autumn days are followed by chill nights, as the Old Sun God returns to the embrace of the Goddess."
(Above taken from this page).