Because one tenet of Neopaganism is that all of nature is cyclical, the passing of time is also seen as a cycle, a wheel which turns and turns. The course of birth, life, decline, and death that we see in our human lives is echoed in the seasons. The eight Sabbats are religious holidays that celebrate the passing of the year.
Each Sabbat also symbolizes a time in the life of the God, who is born from the Goddess, grows to full manhood, mates with her, and reigns as king during the summer. He then declines and dies, rising anew the next year.
The Sabbats, with the traditional dates of their celebrations, are:
Neopagans in the southern hemisphere usually celebrate the Sabbats on the opposite dates of the year (6 months apart from the northern dates), in order to follow the cycle of seasons where they live; i.e. an Australian Neopagan would celebrate Samhain on May 1, when a Canadian Neopagan would be celebrating Beltane.
There are similarities between many Christian holidays and the Sabbats that predated them. It was not at all uncommon for Christian missionaries and priests to adapt local Pagan practices for Christian use. For example, Christmas is today celebrated on December 25 because that was once the date of the winter solstice and hence the date for the celebration of Saturnalia and other forms of Yule.