The traditional Wheel of the Year for Wicca start on Samhain, although the New Year for Greek Wicca starts on Ostara with the return of Persephone.
The year cycles through the phases of the Moon, and the Solar Equinoxes and Solstices.
I started this page with the first 8 traditional Sabbats, which will link to individual pages that explain the history and traditions of each holy day. The next Wheel of the Year page will be the Greek Wicca Wheel, which has an additional 8 holidays specifically for Greek Wicca.