There are eight Wiccan sabbats, which are divided into four High Holy Days and four Lesser Holy Days. Wiccans also celebrate the phases of the moon, and any such gathering is called and Esbat. The four High Holy Days are Samhain (Nov. 1), Imbolc (Feb. 1), Beltane (May 1), and Lughnasadh (August 1). The four Lesser Holy Days correspond with the solstices and equinoxes, so their dates vary each year; Ostara (the spring equinox) falls between March 20 and 22, Midsummer (the summer solstice) falls between June 20 and 22, Mabon (the autumnal equinox) falls between Sept. 20 and 22, and Yule (the winter solstice) falls between Dec. 20 and 22).
Samhain is often referred to as the Witch's New Year because it's the official end of the Light Half of the year, and therefore the beginning of the Dark Half of the year. Samhain is also the time of the year when we harvest the last of the crops and get ready for winter. Because Samhain marks the end of the harvest season, it also represents the beginning of the year to come.
Some Wiccans express this time of the year with the metaphor of the Oak King being overcome by the Holly King. The Oak King is embodies the young and virile king of summer and the Holly King embodies the lord of the underworld and winter. The Oak King loses the battle and temporarily leaves this world while the Holly King rules until he [the Oak King] is born again at Yule.