Plain and simple, a Sabbat is one of eight high holidays observed by Pagans and Wikkans. These celebrations were done in observance of the changes in the yearly cycle. The ancient cultures were close to the land and their survival was closely linked to the yearly progression of nature. The most frequently occurring cycle observed was that of the moon phases. This accounts for the lunar-related Sabbats. The other lest frequently occurring cycle was that of the sun. The solar-based Sabbats are reflected in this cycle.
There are eight Sabbats observed are: Samhain (Celtic New Year), Yule (Winter Solstice), Imbolg (Candlemas), Ostara (Spring Equinox), Bealtaine, Litha (Summer Solstice), Lughnasadh (named for Lugh), and Mabon (Autumn Equinox). Those holidays which aren't either a solstice or equinox observation are the lunar holidays. One other lunar observation is the Esbat which will be the subject of another article.
As noted, Samhain is the Celtic New Year. It is believed that the veils between the world of man and the etheric world is its thinnest at this time. It is or should be observed at Midnight of the 31st of October. Divination is its most accurate and potent at this time. Also, many people celebrate the Feast for the Dead at this time. A meal is prepared and shared during which the participants remember those who have passed on that they wish to be close to.
Yule (22d December), or the Winter Soltice, is most recognizable for its similarity to the Christian holiday of Christmas or Christ's Mass. In fact many holidays observed today are reflections of older celebrations from the old religions. On this holiday, the rejoicing at the rebirth of the Sun God is the central theme. It's no coincidence that the birth of the Christ is celebrated on the same day. As has been the norm throughout history, when one culture subverts another, the deities, and sacred days are either recopied to reflect the incoming regime's belief system or they are totally banned. The Goddess is seen in her Life in Death aspect at this time. The earth dies at the Winter Solstice to be reborn again at Ostara.
