The early Celts/Druids based their Zodiac on a 13 month lunar year and represented each month by a Sacred Tree. The Celts were a society that had an oral tradition. Much of their early spiritual history comes to us filtered through Generations of Christian Monks, who clearly recorded history with their own particular agenda. Because of this there are a number of opinions on dates and details of the Druid Tree Zodiac. Following is one version. Searching out the various theories offers an interesting and dynamic study for anyone willing to put in the effort.

Dec 24 - Jan 20 The Birch
To the Druids, the Birch (often referred to as the "Lady of the Woods" due to its grace and beauty) represented renewal, rebirth and inception, since it was the first tree to come into leaf after the Winter Season.
The Birch gemstone is Rock Crystal, a naturally-occuring substance with the ability to render invisible light visible through the means of refraction. The Druids are said to have used Rock Crystal to make themselves invisible so that they might travel undetected. If held in both hands, it was believed to induce serenity and peace of mind, as well as sharpening the mental processes. An aura of mysticism and magic continues to surround the Crystal and it may be many centuries before all its secrets are truly revealed.
The Birch deity is Lugh, also known as the "Shining One." Lugh was a Hero God whose symbol in Wales was a White Stag and whose sacred symbol was a spear. Always accompanied by two Ravens, Lugh is sometimes depicted as having only one eye.
Birch Animals are Golden Eagle and The White Stag.

Jan 21 - Feb 17 The Rowan
Rowan trees were planted near doors and gates to ward off evil influences and branches were attached to barns in order that the cattle housed inside would be protected from misfortune. This tree was also believed to guard the gateway to the spirit world and its boughs often used for dowsing and deflecting spells.
The Rowan gemstone is the Peridot, also known as "Olivine," the "Evening Emerald" and, in ancient times, called "Chrysolite." Its name derives from the Greek meaning "golden stone." It is a transparent gem of a rich, bottle-green shade which contains a golden inner glow. The Egyptians and Babylonians used it as a protective amulet against evil and it was often used to decorate medieval churches.
The Rowan deity is Brigid, Celtic Goddess of Fertility and Poetry. Daughter of the Dagda, Brigid was a major Celtic pastoral deity and one of the great Mother Goddesses of Ireland.
Rowan Animals are Crane and The Green Dragon.

Feb 18 - Mar 17 The Ash
The Ash was a sacred chieftain tree. he wood of the Ash was thought to be enchanted and was used by the Druids to fashion wands and spears. Its twigs were placed in circles as a protection from snakes, while witches would use the timber as handles for their broomsticks.
The Ash gemstone is Coral, also known as the "Forest of the Sea." It is formed from an animal skeleton and thus, is not a true gemstone, but an ocean jewel. One of the most ancient of gem materials, Coral has been used for adornment since prehistoric times. Once holding a special value for sailors and travelers, it was thought to possess the power of allowing the safe crossing of rivers and seas.
The Ash deity is Gwyddion, son of the Goddess Don. In Welsh Mythology, the Children of Don were the deities who represented the Sky and the people of Cymru referred to the Milky Way as the "Castle of Gwyddion."
Ash Animals are Adder, Seagull and Sea Horse.

Mar 18 - Apr 14 The Alder
The Alder was believed by the Druids to link both male and female principles, thus helping to create a balance between the two within each individual. It is also associated with courage and represents the evolving spirit.
The Alder gemstone is the Ruby, a variety of the Corumdum family (basically aluminum oxide, of which the Sapphire is also a member), which is next in hardness to the Diamond. The Ruby was once considered a powerful amulet, affording protection from disaster, plague and pestilence and promoting a cheeful disposition upon its wearer.
The Alder deity is Bran the Blessed, God of the Spirit World, Celtic Raven God and Welsh God of the Underworld. He is also the God of Prophecy, The Arts, War and Writing, among others, and the patron of bards, minstrels and musicians.
Alder Animals are Fox, Bear and Hawk.

Apr 15 - May 12 The Willow
The Willow symbolizes the female and rhythms of the circle. This tree was sacred to the Moon and, in Celtic lore, the Universe was hatched from two crimson serpent eggs (which contained the Sun and the Earth) hidden among the boughs of the Willow.
The Willow gemstone is the Moonstone. The Romans believed that the gem enclosed the image of Diana, their Moon Goddess, who represented the "moonlight splendor of night." It was said to have the power of bestowing wealth, victory and wisdom upon its wearer.
The Willow deity is Cerridwen the Moon Goddess. Cerridwen was associated with death, fertility, regeneration, inspiration, magic, astrology, herbs, science, poetry, spells and knowledge. Cerridwen is often equated with Hecate, the crone of Greek mythology and also to the Greek Muses, but in a more violent and dark form.
Willow Animals are Adder, Hare and Sea-serpent.

May 13 - June 9 The Hawthorn
The Hawthorn was a symbol of psychic protection due to its sharp thorns. It was also generally seen as a tree which brought good luck to the owner and prosperity to the land upon which it stood. It belongs to the trilogy of sacred Irish trees (the other two being the Oak and the Ash).
The Hawthorn gemstone is the Topaz, a common, semi-precious, transparent gemstone that has been used in jewelry making for centuries. Valued since ancient times as a powerful healing agent, the Topaz is said to aid in affairs of the heart and lend a gentle nature to its owner, as well as possessing magical and occult powers.
The Hawthorn deity is Govanna, custodian of celestial fire and higher powers of the mind. He was also known as the provider of the Fled Goibnenn, a sacred feast associated with brew-crafting and the designated God of Jewelry-Making, Fire, Metalwork and, not surprisingly, the art of Brewing.
Hawthorn Animals are the Bee and the Owl.

Jun 10 - Jul 7 The Oak
The Oak was the principal sacred tree of the Druids, symbolizing truth and steadfast knowledge as well as the turning of the year. During this time, Druids would carve a circle in the tree for protection against lightning.
The Oak gemstone is the Diamond, a sparkling precious stone, white or blue in color, also known as the "King of Crystals" and "Stone of Invulnerability." The Diamond is the hardest substance known in nature. Traditionally, it is the symbol of purity and innocence and is considered luckiest when worn on the left side of the body.
The Oak deity is Dagda, the Father of All Gods, also known as the "Good God" and the Irish Lord of the Land. Ancient with grey hair, Dagda was grounded and simple in every way. He is usually depicted as wearing a brown, low-necked tunic, a short hooded cloak and horse-hide boots with the hairy side outward.
Oak Animals ae the Wren, Otter, Hawk and White Horse.

Jul 8 - Aug 4 The Holly
In the Ogham, it was stated that the Holly was "best in the fight," since it helped balance both the positive and negative aspects of the self, thus revealing a new direction. It was believed to restore lost energy, bestowing the strength needed to continue toward a resolution.
The Holly gemstone is the Red Carnelian, a translucent form of Chalcedony Quartz with a uniform red, reddish-orange to reddish-brown color. Its hue comes from iron impurities in the quartz structure. To change the color of a Carnelian from brownish to red, the stone may be left in sunlight. The name of the gem is derived from the Latin word cornum which means "berry" or "cherry."
The Holly deity is Danu (also known as Dana or Anu), the Goddess after whom the Tuatha De Danaan were named and the Mother Goddess of Irish mythology. Danu was remembered in Celtic tradition on Beltane (May 1) when worshippers carried torches up hills as part of a ritual that included the purification of cattle and sheep and the blessing of the Earth for new crops.
Holly Animals are the Cat and Unicorn.

Aug 5 to Sep 1 The Hazel
The Hazel was considered to be the Tree of Wisdom and to fell one was once a crime punishable by death. It was believed that magickal skills and knowledge could be gained from eating Hazel nuts, which are the emblems of concentrated wisdom. In Irish folklore, the Hazel tree was the home of Bile Ratha, the poetic fairy.
The Hazel gemstone is the Amethyst (sometimes known as "Rose d'France" or the "Stone of the Seventh Ray"), a transparent stone and the most valuable (and probably best known) of the Quartz family. Thought to increase spirituality, it was a symbol of sincerity, security and peace of mind. Accoridng to ancient belief, wearing Amethyst or drinking from an Amethyst chalice would prevent intoxication.
The Hazel deity is Oghma (also known as Ogma), son of Boann and The Dagda. The God of Communication and Writing, as well as Literature and Eloquence, Oghma is said to have invented the Ogham Alphabet and made a gift of it to the Druids.
Hazel Animals are Crane and Salmon.

Sep 2 - Sep 29 The Vine
In the cool Celtic climate, the GrapeVine required much care in order to bear fruit and became symbolic of sacred knowledge and spiritual initiation, as well as a symbol of sensuality and emotions. The Vine of the Ogham Alphabet is the GrapeVine.
The Vine gemstone is the Emerald, a sparkling-green precious stone of the Beryl group, its color caused by small amounts of chromium and enhanced by traces of iron. One of the most expensive jewels, the Emerald is, in some instances, more valued than the Diamond, with the highest quality stones being a transparent grass-green.
The Vine deity is Branwen, also known as "Fair Bosom," sister of Bran the Blessed and wife of the Irish King Mathowch. Branwen was the daughter of Llyr, Lord of the Sea and literary ancestor of Shakespeare's King Lear. Otherwise called "Venus of the Northern Seas," Branwen was one of the three matriarchs of Britain and the Welsh Goddess of Love, Sexuality and the Sea.
Vine Animals are Lizard, Hound and White Swan.

Sep 30 - Oct 27 The Ivy
The power of the Ivy lies in its ability to cling and bind, making it a potent symbol of determination and strength to the Druids. Ivy has been known to strangle trees and was once a portent of death and spiritual growth. Being evergreen in nature, the Ivy represented the perennial aspects of the human psyche.
The Ivy gemstone is the semi-precious Opal, also known as the "Queen of Gems" and "Gem of Hope." The early Greeks believed the Opal bestowed powers of foresight and prophecy upon its owner while in Arabian folklore, it is said that the stone fell from heaven in flashes of lightning.
The Ivy deity is Arianrhod whose name means "Silver Circle." She is also known as "Star Goddess," "Full Moon Goddess" and "Virgin Goddess of Reincarnation," among others. Her palace was called "Caer Arianrhod," otherwise known as the Aurora Borealis. She was Keeper of the Silver Wheel of Stars...a symbol of time and/or karma.
Vine Animals are Bear, Goose and Butterfly.

Oct 28 - Nov 24 The Reed
The Druids believed the Reed to be a tree because of its dense system of roots. Cut reeds were used as pens and symbolized wisdom and scholarship. Identified with the submerged or hidden Dryad, the Reed was representative of the mysteries of death. It was associated with being a saviour and a custodian as well as a symbol of royalty.
The Reed gemstone is the Jasper, also known as the "Supreme Nurturer." The stone is an opaque, impure Chalcedony, usually red in color but may also be found in brown, green, greyish-blue and yellow. It is also often multi-colored. A very pretty and inexpensive earth stone, Jasper is usually worn in the form of beads or cabochons.
The Reed deity is Arawen (or Arawn), King of the UnderWorld, also known as "King of Hell" and "God of Annwn." His name means "silver-tongued." Arawen fought alongside Bran in the Battle of the Trees against Amathaon and Gwyddion. Like most Gods of the OtherWorld, Arawen was a master hunter who rode a pale Horse, followed by a pack of white, red-eared Hounds.
Reed Animals are Owl and The White Hound.

Nov 25 - Dec 23 The Elder
To the Celts, the Elder (also known as the "Tree of Faeries," "Old Gal," "Pipe Tree" and "Lady Ellhorn," among others) was the symbol of both death and rebirth. Its twigs were said to enable the wearer to see spirits and experience visions. Justice was dispensed by the Druids beneath this tree. Much like the Winter Solstice itself, it highlighted a time of evolution.
The Elder gemstone is the Jet, sometimes called "Black Amber." It is an organic jewel rather than a gemstone, being a variety of Lignite, formed from the wood of cone-bearing plants which have fossilized...a type of coal intermediate between peat and anthracite which is so hard and uniform that it may be carved and polished to look like black glass, even though it radiates no light.
The Elder deity is Pryderi, son of Pwyll Penn Annwn and the Goddess Rhiannon. According to legend, Pryderi was stolen away as a newborn infant by a nameless fiend who, upon a horse-thieving expedition, dropped the babe once more into the world when it was struck by a blow from the Guardian of the Horses.
Elder Animals are Raven, The Black Horse and Badger.

Thanks to www.novareinna.com for this information.