Druid signs are also known as Celtic astrology or the Celtic zodiac. The 13 signs are based on trees held sacred by the Druids: Birch, Rowan, Ash, Alder, Willow, Hawthorn, Oak, Holly, Hazel, Vine, Ivy, Reed and Elder.
The Druids, who were the priestly class of the Celtics, created Celtic astrology around 1000 B.C. The Druids believed that each of the 13 trees expressed spiritual characteristics that were associated with months of the year. There are 13 Celtic signs because the definition of a month in Druid culture is based on the number of full moons in a year. Druidism is a religion based on nature and ancestry and can adapt to a large variety of spiritual beliefs.