The Celtic Tree Calendar is a calendar with thirteen lunar divisions. Most contemporary Pagans use fixed dates for each "month", rather than following the waxing and waning lunar cycle. If this was done, eventually the calendar would fall out of sync with the Gregorian year, because some calendar years have 12 full moons and others have 13. The modern tree calendar is based on a concept that each letter in the ancient Celtic alphabet corresponded to a tree.
Ivy Moon

Image © John Foxx/Getty ImagesSeptember 30 - October 27: As the year comes to a close and
Samhain approaches, the Ivy moon rolls in at the end of the harvest season. Ivy often lives on after its host plant has died -- a reminder to us that life goes on, in the endless cycle of life, death and rebirth. The Celts called this month
Gort, pronounced
go-ert. This is a time to banish the negative from your life. Do workings related to improving yourself, and placing a barricade between you and the things that are toxic to you. Ivy can be used in magic performed for healing, protection, cooperation, and to bind lovers together.
Reed Moon
October 28 - December 23: Reed is typically used to make wind instruments, and this time of year, its haunting sounds are sometimes heard when the souls of the dead are being summoned to the Underworld. The Reed Moon was called Negetal, pronounced nyettle by the Celts, and is sometimes referred to as the Elm Moon by modern Pagans. This is a time for divination and scrying. If you're going to have a seance, this is a good month to do it in. This month, do magical workings related to spirit guides, energy work, meditation, celebration of death, and honoring the cycle of life and rebirth.
Elder Moon
November 25 - December 23: The
winter solstice has passed, and the Elder moon is a time of endings. Although the Elder can be damaged easily, it recovers quickly and springs back to life, corresponding to the approaching New Year. Called
Ruish by the Celts (pronounced
roo-esh), the month of Elder is a good time for workings related to creativity and renewal. It is a time of beginnings and endings, births and deaths, and rejvenation. Elder is also said to protect against demons and other negative entities. Use in magic connected to Faeries and other nature spirits.