Wiccans observe two natural cycles as part of their worship. These two cycles are the Phases of the Moon and the Wheel of the Year.
The Wheel of the Year is celebrated on the eight Sabbats. These are based on Solar events, such as the Solstices and Equinoxes, with "cross-quarter" festivals of the Celtic calendar. These eight Sabbats are listed below, with some of the more common names and approximate dates. Note that different Traditions may assign different dates and names to these celebrations.
The customs, rituals, and mythologies for each of these Sabbats is far too complex for a single webpage. There are several books that give a good introduction to these celebrations; but the best way to learn about them is to live them!