 Graves also argues that the names of the Ogham letters in the alphabet used in parts of Gaelic Ireland and Britain contained a calendar that contained the key to an ancient liturgy involving the human sacrifice of a sacred king, and, further, that these letter names concealed lines of Ancient Greek hexameter describing the goddess.Graves' "Tree Calendar" has no relation to any historical Celtic calendar.His interpretations rather rely on the book "Ogygia" by the 17th-century bard Roderick O'Flaherty.