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The White Goddess at http://www.thewhitegoddess.co.uk/the_wheel_of_the_year/mabon_-_autumn_equinox.asp
All of these are great sites from which to learn.
In the end, Mabon is a spiritual event that is all about giving thanks, abundance, and balancing your life out. Give thanks for your harvests whether physical or spiritual. Take a journey into your past. Look not just over the past year, but over your lifetime. Make your plans for your future, for Mabon is a time to finish old plans and projects. It is, also, a time for planting seeds for the future, whether it be for a new business or a change in your life, physically or spiritually. Help others by giving.
Happy Mabon! Blessed Be!