Witches follow the old Celtic agricultural calendar and the sabbats are seasonal celebrations. They are the holy days of a nature religion when we express our gratitude for the beauty and bounty of the earth. They have nothing to do with the devil or evil-doing.
The four main sabbats are:
October 31 Hallowe'en, which we call Samhain, an Irish word pronounced (more or less) Sow (as in female pig)-wen. This marks the end of the old year and the "death" of the growing season. By association it became a time to honour departed loved ones and also the ancestors who have gone before us. (Christians adopted this practice and called it All Souls Eve.) The New Year then begins on November 1.
Jan. 31 / Feb. 1 Imbolc, or Imbolg (Welsh for in-the-belly, i.e. the earth "pregnant" with the spring), Oimelc (ewe's milk) or Lady Day. This marks the lambing time and the growing strength of the sunshine that will soon bring the summer. (Remember, this was based on British experience, not Canadian!)
April 30 / May 1 Beltane, (meaning bright fire), May Eve/May Day. In England this was the time of flowers and preparations for sowing the grain. In this part of the world we still celebrate it as a joyful occasion, namely the end (almost certainly) of winter. Yes, we dance around the maypole, an old rite for the fertility of the crops.
July 31 / August 1 Lammas, meaning "loaf-feast", for the bread made from the first grain of the season. Or Lughnasad, after Lugh the sun god, who is already decreasing in strength, but who has put his energy into the crops to sustain us for another year.
Those are the major sabbats because they mark the end of one season and the beginning of another. Between are the four minor sabbats, which occur on the solstices and equinoxes. They are Yule on or about December 21, Ostara on or about March 21, Midsummer on or about June 21, and Harvest on or about September 21. Witches celebrate the sabbats outside when possible, usually with a bonfire; otherwise we meet in someone's home. See our paper on rituals for the general outline of what we do on these occasions. Following the ritual we have a feast, preferably of local foods appropriate to the season.