Yule, the winter solstice, is the longest night of the year. The Horned One, who is NOT the devil, is the Wiccan God of the Sun and brings longer days during His rule.
As for the three-fold law, Witches do believe that what we do comes back to us. Our acts of kindness, forgiveness, and love are returned to us three times. Our acts of nastiness, bitterness, and hate are also returned to us three times.
This is actually a pretty good question... thanks for asking!