The Wheel of the Year is our calendar. It is comprised of eight holidays known as Sabbats, starting with Samhain. It is called The Wheel of the Year because we view time as cyclical, not linear - the world lives and dies and lives again. The Sabbats are solar festivals following the solar year, and so their mythology emphasizes the life cycle of the God, whom we associate with the sun.
The Wheel of the Year is not historical, but more a modern adaptation of historical practices. The Sabbats take their names and some of their purposes from a variety of pagan holidays from a variety of cultures - the major Sabbats are more Celtic-based, while the minor Sabbats are more Germanic-based. The defined life cycle of the God and Goddess throughout the year is a Wiccan concept.
The Wheel is split into two halves for summer and winter, with the divisions occurring at Samhain and Beltane. The Sabbats are divided into two groups: the Major Sabbats and the Minor Sabbats, which correspond to the equinoxes and solstices:
Litha or Midsummer
The strict assignment of major Sabbats to specific dates is modern. Samhain, for example, was a celebration of the end of the harvest. The harvest wasn't completed on the same day every year. Some years the crops took longer to mature than others, or fewer hands were available to assist in the harvesting. Another thing to note is that in older times, the days began at sundown. Therefore, "November 1" is actually the night of October 31 through the day of November 1.