The Celtic tree calendar is based on the lunar year beginning after the winter solstice. It is thought to have been devised or popularised in the nineteenth century by the author Robert Graves as part of a Celtic revival. There are roughly 13 lunar months in the calendar which begin and end with the new moon, and each lunar month is represented by a tree. Five ‘solar’ trees cover a larger portion of the year, usually about 2-3 months each, like the seasons.

Celts had a profound connection with trees and believed and that each tree had its own sacred purpose. The Oak (duir) was the most sacred tree of all representing qualities of strength, endurance, fortitude and loyalty.

The Celtic Tree Calendar may have been derived from the ancient Celtic alphabet where each letter corresponded to a sacred tree: the first letters of the names of lunar trees (in Gaelic) are consonants, and the five solar trees are vowels. These letters corresponded to strokes and dots – an ancient form of Irish writing called Ogham.