The Ogham
The Ogham is an early medieval alphabet used primarily to write the early Irish language, it is thought that it may have originated much earlier. The ogham alphabet originally consisted of twenty distinct characters and each of these represents a Tree. The twenty trees with their relevant ogham symbols are listed below.
The chart on the left is taken from HERE, This site will also give you more in depth information about each tree.
Celtic Tree Calendar
Although named The ‘Celtic Tree Calendar’ there is no evidence that the celts named months using trees from the Ogham and it was not until the 18th century that this idea was popularised. We use it today because trees are sacred to druids and as such it is good to use taking the different properties of each tree and attaching them to the appropriate month and using it as a sort of tree astrological system. Here is the calendar as documented.
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Beth (Birch) December 24 to January 20
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Luis (Rowan) January 21 to February 17
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Nion (Ash) February 18 to March 17
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Fearn (Alder) March 18 to April 14
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Saille (Willow) April 15 to May 12
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Uath (Hawthorn) May 13 to June 9
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Duir (Oak) June 10 to July 7
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Tinne (Holly) July 8 to August 4
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Coll (Hazel) August 5 to September 1
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Muin (Vine) September 2 to September 29
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Gort (Ivy) September 30 to October 27
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Ngetal (Reed) October 28 to November 24
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Ruis (Elder) November 25 to December 22
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Mistletoe represents December 23 as it is not ruled by any tree, it is the traditional day of the proverbial "Year and a Day"