Today is the day of the Autumnal Equinox. It is the second of the three harvest festivals - the first being Lughnasadh and the third being Samhain. Mabon is a pretty new holiday, though it has been honored throughout time as the equinox.
In modern paganism, the Autumnal Equinox - or Mabon - is a day of perfect balance, a time when we honor both the light and dark aspects within ourselves and, by extension, the world. We honor both the light and dark aspects of divinity, and spend this day looking towards the dark part of the year - as from this day forth we will begin to get a bit darker each day during winter.
Mabon was so named to fit in with the more celtic sounding names of the other sabbats by Aidan Kelly in the 1970s. Mabon is the name of a Welsh god, and for a bit of history I turn now to the Encyclopedia of Mythology by Arthur Cotterell:
Mabon, son of the Welsh divine mother Modron, was said to have been abducted when only three nights old and imprisoned in Gloucester. However, since only he was able to control the hound which Culhwch needed to win the hand of Olwen, an expedition was mounted to release Mabon. Once free, he duly helped to capture the wild boar Twrch Trwyth with the aid of the hound and to take from between the boar's ears the razor that Olwen's father had demanded. Apart from adventures like this, the actions of Mabon are uncertain, suggesting that he may have ben a former god, possibly Maponos, a Celtic god of youth, who was incorporated in Welsh mythology as a warrior once his worship was all but forgotten. The Romans knew of Maponos, whom they equated with Apollo, the god of prophecy.
Another popular myth told at this time is that of Demeter and Persephone. Long story short: Persephone is the daughter of Demeter (grain goddess) who was either stolen away or ran away with - depending on your myth - Hades (death god). Demeter was pissed and wouldn't let anything grow until good ol' Zeus (king god/sky god) stepped in and suggested a compromise in which Persephone spends 6 months with Hades and 6 with Demeter. Thus Persephone is a Spring goddess, and the reason we have a barren winter is because that's when she is in the underworld.
Have a yummy meal with your family that includes second harvest fruits - like grapes - and perhaps a little wine. Honor the balance of the light and dark in the world an in yourself. Also, a fun little tradition is to get a cornucopia - or adequate substitute - and write down all the things you're thankful for, bless these, and bury them in the earth so that they continue to grow. The blessing I use is below.
Love and Lyte,
Lord and Lady, we thank and praise you for our many blessings.
We welcome the coming months, and celebrate this day of balance, when Lord and Lady are equal.
I bless these messages with gracious, peaceful energy, and I pray they are well received.
May our energies be sent back into the Earth so that the coming harvest will be bountiful.
We honor your blessings and vow to share them with our community.
We rejoice in this season of bounty, and the turning of the Great Wheel of the Year!
Lord and Lady, Blessed Be!