The Celtic Tree Calendar/Alphabet is traditionally known as the Ogham (pronounced oh-am).
The origins of this script is unknown, considering much of Druidic/Celtic tradition and wisdom was not physically preserved in texts or documents, and was essentially lost over time. What scholars do know comes from manuscripts and documents produced by outside sources who visited the Celts/Druids and recorded their own observations of the traditions and practices. Most of the recorded information came for secondary sources and modern recording of the oral traditions. Scholars do know the basic technical aspects and symbolism of the ancient script.
The Ogham script is made up of symbols of straight lines, known as flesc (flayshk), spaced along a central stemline. Each symbol represents a particular letter, and each letter is named after a tree or plant, or after a natural element such as lightning or the sea.
The fifteen Ogham letters are represented by fifteen different trees, and the corresponding calendar is based on the thirteen-month lunar calendar, which begins on November 1. Each month is characterized by a tree, which has special teachings, guides, totems and deities associated with each specific tree.