The holidays of the Wiccan faith are arranged to match the cycle of the year.
Samhain (October 31): Celebration of the ancestors; commemoration of the dead.
Yule (December 21): Rebirth of the Sun God. Winter Equinox,
Imbolc (February 1): Increase of the sun's power; new beginnings; healing.
Ostara (March 21): An agricultural/planting holiday; spring equinox
Beltane (May 1): A fertility holiday; celebration of life.
Litha (June 21): the Sun God is at his full power.
Lughnassadh (August 1): A harvest holiday (corn/grain). The Sun God dies and becomes the Lord of Shadow.
Mabon (September 21): A harvest holiday (apples/fruit).
The liturgical calendar is referred to as the Wheel of the Year. In addition to the eight holidays (Sabbats), Wicca also recognizes thirteen Full Moon events, called Esbats. The former are solar holidays while the latter are lunar holidays.
In the Wiccan faith "light" does not automatically equate "good", nor does "dark" equate "evil". Wicca is a religion that understands the necessary balance of forces. There cannot be shadow without light or death without life.
Samhain, Imbolc, Beltane, and Lughnassadh are considered "high" Sabbats. Yule, Ostara, Litha, and Mabon are considered "low" Sabbats.