Modern, not ancient

The atlas is clear that the Celtic tree calendar is a twentieth-century creation, not a calendar kept by the ancient Celts: it rests on Graves's personal and much-criticised interpretation rather than on evidence of Iron Age Celtic practice, though it draws on the genuine medieval Irish tree-lore attached to the ogham. The distinction is a service to both the modern system and the real history it borrows from.

The atlas gives each tree-sign its month, its tree, and its associated character. The ogham letters and their medieval tree-lore that Graves drew on are treated in their own categories, and the calendar belongs among the Modern Revivals.