The Wheel of the Year divides the solar cycle into eight festivals spaced roughly six and a half weeks apart, alternating two kinds of observance. The quarter days are astronomical: the two solstices, kept as Yule and Litha, and the two equinoxes, kept as Ostara and Mabon. Between them fall the cross quarter festivals, drawn from the old Irish reckoning of the year: Samhain at the start of November, Imbolc at the start of February, Beltane at the start of May and Lughnasadh at the start of August. Together they give practitioners a festival never more than a few weeks away, and the wheel is usually drawn as exactly that, a circle with eight spokes, turned by the sun.
In Wiccan practice the sabbats carry a seasonal mythology of the God and Goddess through birth, growth, harvest and death, though the tellings vary widely between traditions and much of modern Paganism keeps the festivals without that frame. Alongside the sabbats, many groups keep esbats, working meetings set by the moon, most commonly at full moon, so the calendar runs on two wheels at once, solar and lunar. The atlas holds each sabbat as its own entry with its own history, because the eight festivals have eight quite different pedigrees, and links them here as members of the single cycle that modern practice joins them into.