Peter Chadwick LRPS / Moment / Getty Images
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 letters in the ancient Celtic Ogham alphabet corresponded to a tree.
Although you don't have to follow a Celtic path to celebrate the Celtic tree calendar months, you'll find that each of the themes in the Celtic tree months ties strongly to Celtic culture and mythology.
It's also important to note that there is no proof that the Celtic tree calendar actually originated with early Celtic peoples. Joelle of Joelle's Sacred Grove says,
"The lunar tree calendar of the Celts has long been a source of controversy among Celtic scholars. Some even claim it was never a part of the old Celtic world, but was an invention of author/researcher Robert Graves. The Druids are generally given credit by other researchers for creating this system. There seems to be no scholarly evidence to prove otherwise, yet many Celtic Pagans feel that the system pre-dates the time of Druidic influence over Celtic religious matters. It is probably reasonable to believe that the truth lies somewhere in between these three extremes. It is most likely that the tree system was in place, with minor regional variations before the time of the Druids who experimented with it, discovered the magical properties of each tree, and codified all the information into the system we have today."
The Rowan Moon is associated with Brighid, the Celtic goddess of hearth and home. Honored on February 1, at Imbolc, Brighid is a fire goddess who offers protection to mothers and families, as well as watching over the hearthfires. This is a good time of year to perform initiations (or, if you're not part of a group, do a self-dedication). Known by the Celts as Luis (pronounced loush), the Rowan is associated with astral travel, personal power, and success. A charm carved into a bit of a Rowan twig will protect the wearer from harm. The Norsemen were known to have used Rowan branches as rune staves of protection. In some countries, Rowan is planted in graveyards to prevent the dead from lingering around too long.
Wigington, Patti. "Celtic Tree Months." ThoughtCo, Oct. 16, 2018, thoughtco.com/celtic-tree-months-2562403.