Most pagans celebrate 8 holidays each year, called sabbats. This cycle of holidays is generally referred to as the Wheel of the Year.
The Wheel is made up of the four equinoxes and solstices and four cross-quarter days. The solstices and equinoxes are the minor or Lesser sabbats and the cross-quarter days are the major or Greater sabbats.
The major sabbats are based on old (generally) Celtic folk festivals. These days transect the quarters made by the minor sabbats. The minor sabbats are celebrated on the astronomical baginnings of the seasons, which is why I have "around the date" for some of them, they can change slightly from year to year. The equinoxes are when day/night are equal and the solstices are the longest day/night of the year. Celebrations usually start at sundown on the previous day.
In some traditions, based on the Celtic year, the Wheel represents the life / death cycles of the Holly King and the Oak King. The Oak King rules the light half of the year and the Holly King rules the dark half. There are two schools of thought for this representation. In the first, they Oak and Holly Kings battle twice a year, at Yule and Midsummer (Litha). At Yule the Oak King prevails, the Holly King dies and is reborn at Midsummer. At Midsummer, the Holly King prevails and the Oak King dies to be reborn at Yule. For the second, both live simultaneously, with the Holly King waning in power at Yule and the Oak King waning at Midsummer.
A more Wiccan variation of the Wheel involves the God and Goddess. The Goddess gives birth to the (Sun) God at Yule, who grows stronger and brings the return of Spring. They unite at Beltane, when the seed of the new God is planted. The God is sacrificed with the crops and animals at Samhain and prepares for his rebirth at Yule.
Click the links on the right for more information about each Sabbat.