This is what Byzant Mystical has to say about Mabon:

Mabon is the solar festival that marks the transition from the light to the dark half of the year: day and night are of equal length. On this day, the sun rises due east and sets due west. The autumn quarter of the year runs from Lughnassadh to Samhain, so Mabon marks the mid-point of autumn. By Mabon, the land is showing clear signs of the journey towards winter - leaves are beginning to turn and birds are gathering for migration.

Mabon is the time of the second harvest, when fruits are ready for collection. In Celtic mythology, Mabon was the Young God, abducted and imprisoned, only to return at a later date. This is thus an appropriate title for the day on which darkness gains the upper hand over light until the following equinox, Ostara. Mabon is the point at which, conceptually at least, the Sun enters the sign of Libra, the Scales or Balance - the most appropriate sign for this day of perfect balance between darkness and light.

Mabon is a time to consider which aspects of your life you wish to preserve and which you would prefer to discard. It marks a time of thankfulness, equality and balance when you should try to appreciate and enjoy the fruits of your labors. The dark half of the year brings a greater emphasis on reflection, rest and planning, and Mabon is the point at which these influences begin to be felt.