Samhain (pronounced saw-wane) is the end of the old year and the beginning of the new according to the witches calendar of the cycle of thre year. We have seen the birth of spring, and the growing season, and finaly the harvest. Birth-Life-Death.
The cycle has finaly come full circle. So at Samhain we celebrate not only the end of this cycle, but also the knowledge that this cycle will repeat itself, and Life will begin anew.
In some traditions, it is also a celebration of those who have gone before us, and that which they have done, to make this earth what it is for us.