The Wheel of the Year
The eight Sabbats each mark an important point in nature's annual cycles. They are depicted as eight evenly spaced spokes on a wheel representing the year as a whole; the dates on which they fall are nearly evenly spaced on the calendar, as well. The wheel is comprised of two groups of four holidays each. There are four solar festivals relating to the sun's position in the sky, dividing the year into quarters: the Ostara, Litha, Mabon and Yule. Between these quarter days are the cross-quarter holidays, or fire festivals: Imbolc, Beltane, Lughnasadh, and Samhain.
Note: Most dates in these posts apply to the northern hemisphere. In the southern hemisphere, the seasons are opposite and it's up to you whether you celebrate on the traditional dates developed in the north or on the dates corresponding with your local natural events.