The weather has started to cool and with it the arrival of fall (Sept. 23rd). The Wheel of the Year hails the day as Mabon. Mabon is also known as the Fall, or Autumn, Equinox. Wiccans and Pagans celebrate the day as the beginning of the darker half of the year (as in Ying/Yang). After the Autumn Equinox, days shorten and nights get longer. This triggers a beautiful change in the foliage of trees and shrubs.
The Wheel is celebrated all over the world in many ways. I have included a link to a fun video from Europe. The music is catchy and the folks are dancing in witches costumes for Walpurgis Night, but it's just fun to watch.
The changing of leaves from green to glorious reds, oranges, yellows, organs, and bronzes are looked forward to by hikers and gardeners every year. How does this change happen?
Keeping it simple, the color change in leaves has to do with photosynthesis and chlorophyll. Weather conditions also pay a large part. September starts a cooling and drying trend. The shorter days means less light. Remember that with photosynthesis: the chemical reaction of light, carbon dioxide, chlorophyll, and water creates energy in the form of sugar and releases oxygen through leaves. As the season changes, leaves are "sealed" off from the stem by abscission. Because water can't flow to the leaves, photosynthesis can't occur. Chlorophyll is green but the leaves of deciduous (broadleaf) trees and shrubs also carry other pigments. This allows the other colors to come out and display their beauty.
Yellows, browns, and oranges are due to carotene. Anthocyanins make for red and purple. Tannins also cause the browns and bronze. Leaves gradually change from green to their brilliant fall foliage.
also see "Colorful Fall: article by Ed Brotak in the current issue of Horticulture Magazine.
a more detailed explanation with fall forest color link:
Wolfshager Hexenbrut dance video (copy and paste)