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.