Centuries ago, the witches of the north observed the sabbath by gathering in a cemetery in the shadows of the moon. On one gathering, the Norse goddess, Freya (the origin of "Friday"), came down from her sanctuary in Valhalla and appeared before the group, who numbered 12. Freya gave them one of her cats, completing what came to be known as a witches' coven. Ever since then, a perfectly-numbered coven has comprised exactly 13.
BTW, I'll be posting information and pictures shortly of my 2009 Halloween!