The Celtic Tree Calendar is a 13 month calendar meintioned by Robert Graves in his book The White Goddess. Each month is named after a Tree and has a Rune associated with it. In addition to the 13 months, there is an extra day to bring the number of days up to 365. There is no mention of any leap year rule, even though such must exist for the months to occur (approximately) on their equivalent Gregorian dates (as shown next).
The months are
Name Equivalent Dates Tree Beth Dec 24 - Jan 20 Birch Luis Jan 21 - Feb 17 Rowan Nion Feb 18 - Mar 17 Ash Fearn Mar 18 - Apr 14 Alder Saille Apr 15 - May 12 Willow Uath May 13 - Jun 9 Hawthorn Duir Jun 10 - Jul 7 Oak Tinne Jul 8 - Aug 4 Tinne Coll Aug 5 - Sep 1 Hazel Muin Sep 2 - Sep 29 Vine Gort Sep 30 - Oct 27 Ivy Ngetal Oct 28 - Nov 24 Reed Ruis Nov 25 - Dec 22 Elder Extra day Dec 23
The calendar is used a by neopagans, but there is no record of historical use.