Mabon is the Fall Equinox, named after the Celtic God of the same name. Day and night are in balance, as they were at Ostara, but now we descend into darkness, for we (at least in the Northern Hemisphere – forgive me for being north-centric, the equator is one line I haven’t yet crossed) have winter before us.
The Mother provides us with the last harvest of the year. She will be fully the Crone by the next sabbat (Samhain).
It is not yet cold where we live at Mabon, and it is very likely that we are in the midst of the last gasp of summer in a final heat wave. Here on the East coast of North America, it is also the height of hurricane season, where Mother Nature gets angry and much destruction can occur.
That said, it is a time for celebrating and feasting with the harvest, and also a good time to remember to balance yourself in as many ways as you can manage.
Projects for this holiday can relate to the season at hand: making things with leaves, harvested material, cooking, canning, working in the yard. Projects can also reinforce the other themes: Children can draw pictures of what they have finished (harvested), they can make mobiles, and balance eggs(see Ostara project).
By the way, on the urban legends page, I found out that I have been a devout believer in what is an Urban Legend! Apparently, eggs will balance at anytime, and there is nothing particularly special about the exact moment of equinox as far as the tides are concerned. However, one educator pointed out that you can balance eggs on the equinox to make it special for children.