Celtic tree calendar calculator

The 'Celtic Tree Calendar'

The 'Celtic tree calendar' or 'tree zodiac' divides the year into thirteen lunar months of twenty-eight days, each named after a tree and a letter of the Ogham alphabet: Birch, Rowan, Ash, Alder, Willow, Hawthorn, Oak, Holly, Hazel, Vine, Ivy, Reed and Elder, with a single extra day, 23 December. It is widely presented in modern Pagan books and websites as the ancient calendar of the Druids.

The scheme was in fact devised by the poet Robert Graves in The White Goddess (1948), building on speculative eighteenth-century writers such as Edward Davies and on his own poetic intuition. Graves himself presented it as a 'historical grammar of poetic myth' rather than as documented history. There is no evidence that the ancient Celts used a tree calendar or a thirteen-month year; the Gaulish Coligny calendar (2nd century CE), the only surviving Celtic calendar, is a lunisolar calendar of twelve months with a different structure.

Ogham itself is real: an early medieval alphabet of strokes along a line, used in Ireland and western Britain from about the fourth century for inscriptions on standing stones. Medieval Irish scholars later named many Ogham letters after trees in 'word-oghams', which is where the tree associations come from — although some of the identifications (such as 'vine' in Ireland) are doubtful. The tree calendar is best understood as a modern contemplative and poetic system, and it is presented here in that light.