The Celtic calendar is a compilation of pre-Christian Celtic systems of timekeeping, including the Gaulish Coligny calendar, used by Celtic countries to define the beginning and length of the day, the week, the month, the seasons, quarter days, and festivals.
Continental Celtic Calendar 
The Gaulish Coligny calendar is possibly the oldest Celtic solar/lunar ritual calendar. It was discovered in Coligny, France, and is now on display in the Palais des Arts Gallo-Roman museum, Lyon. It dates from the end of the second century CE, when the Roman Empire imposed use of the Julian Calendar in Roman Gaul. The calendar is made up of bronze fragments, in a single huge plate. It is inscribed in Gaulish with Latin characters and uses roman numerals.
The Coligny Calendar is an attempt to reconcile both the cycles of the moon and sun, as is the modern Gregorian calendar. However, the Coligny calendar considers the phases of the moon to be important, and each month always begins with the same moon phase. The calendar uses a mathematical arrangement to keep a normal 12 month calendar in sync with the moon and keeps the whole system in sync by adding an intercalary month every 21⁄2 years. The Coligny calendar registers a five-year cycle of 62 lunar months, divided into a "bright" and a "dark" fortnight (or half a moon cycle) each. The months were possibly taken to begin on the new moon, and a 13th intercalary month was added every two and a half years to align the lunations with the solar year.
The astronomical format of the calendar year that the Coligny calendar represents may well be far older, as calendars are usually even more conservative than rites and cults. The date of its inception is unknown, but correspondences of Insular Celtic and Continental Celtic calendars suggest that some early form may date to Proto-Celtic times, roughly 800 BCE. The Coligny calendar achieves a complex synchronization of the solar and lunar months. Whether it does this for philosophical or practical reasons, it points to considerable degree of sophistication.
Medieval Irish and Welsh calendars 
Among the Insular Celts, the year was divided into a light half and a dark half. As the day was seen as beginning after sunset, so the year was seen as beginning with the arrival of the darkness, at Samhain (in modern times the 1 November, or for modern Pagans in early November). The light half of the year started at Bealtaine (in modern times 1 May, or for modern Pagans in early May). This observance of festivals beginning the evening before the festival day is still seen in the celebrations and folkloric practices among the Gaels, such as the traditions of Oíche Shamhna (Samhain Eve) among the Irish and Oidhche Shamhna among the Scots.
Julius Caesar said in his Gallic Wars: "[the Gaulish Celts] keep birthdays and the beginnings of months and years in such an order that the day follows the night." Although Caesar says "at night" he specifically does not say "sunset" so we do not know how much the Gauls differed from others in methods of counting from midnight. Longer periods were reckoned in nights, as in the surviving term "fortnight" and the obsolete "se'nnight".
Native Calendar terms in the Celtic languages 
Many calendrical and time-keeping terms used in the medieval and modern Celtic languages were borrowed from Latin and reflect the influence of Roman culture and Christianity on the Insular Celts. The words borrowed include the month names Januarius (Old Irish Enáir, Welsh Ionawr), Februarius (Old Irish Febra, Welsh Chwefror), Martius (Old Irish Mart, Welsh Mawrth), Aprilius (Old Irish Apréil, Welsh Ebrill), Maius (Welsh Mai), Augustus (Old Irish Auguist, Welsh Awst); the names for the days of the week, dies Solis, Lunae, Martis, Mercurii, Jovis, Veneris, Saturni; the terms septimana "week" (Breton sizun, Cornish seithum), kalendae "first day of the month" (Old Irish callann, Welsh calan, Breton kala), tempore "time" (Welsh tymor), matutina "morning" (Cornish metin), vespera "evening", nona "noon" (Welsh nawn), and ôra "hour" (Welsh awr, Breton eur).
A number of native Celtic terms survived the adoption of the Roman/Christian calendar, however:
|Term||Proto-Celtic||Gaulish||Old Irish/Middle Irish||Scottish Gaelic||Manx||Welsh||Cornish||Breton|
|Day / 24-hour period||*latįon||lat (abbreviation, Coligny Calendar)||la(i)the||là, latha||laa|
|Night||*nokWt-, *ad-akWi-(?)||(tri)nox "(3)-night, (decam)noct- "(10)-night-"||nocht, adaig||nochd, oidhche||noght, oie||noeth (in compounds), nos||neth (comp.), nos||neiz (comp.), noz|
|Week (eight nights/days)||*oktu-nokWt- / *oktu-diį-||wythnos "8-nights"||eizhteiz "8-days"|
|Fortnight||*kWenkWe-decam-nokWt-||cóicthiges "15 (days)"||cola-deug (coig latha deug "15 days"||pythefnos "15 days"||pemzektez|
|Month||*mīss-||mid (read *miđ)||mí||mìos||mee||mis||mis||miz|
|Year||*bl(e)id-anī-||b[l]is (abbreviation, Coligny Calendar)||bliadain||bliadhna||blein||blwydd, blwyddyn||bledhen||bloavezh, bloaz|
|Season, Period of Time||*ammn, *ammn-stero-, *ratio-, *pritu-||amman||amm, aimser, ráithe||àm, aimsir||imbagh, emsher||amser, pryd||amser||amzer|
|Spring||*ers-āko "end (of winter)" (alt. *uesr-āko "spring[time]"), *ues-ant-ēn-, *ro-bertiā ("torrent, inundation")||earrach, robarta||earrach||arragh||gwanwyn, (Old Welsh) ribirthi||gwainten||reverzi (Old Breton rebirthi)|
|Autumn||*uφo-gijam-r- "under wintertime", *kintu-gijamo "beginning of winter", *sito-[...] "deer-"(?)||fogamur||foghar||fouyr||cynhaeaf, hydref||kydnyav/kynyav, hedra,||here, diskar-amzer ("falling season")|
|May, May Day||*kintu-samVn- (V=indeterminate vowel) "beginning of summer"||Cétamain||Cèitean||Cyntefin|
|June, Midsummer||*medio-samVn- (V=indeterminate vowel) "mid-summer"||Mithem(on)||Mehefin||Metheven||Mezeven|
In Neopaganism 
In some Neopagan religions, a "Celtic calendar" loosely based on that of Medieval Ireland is observed for purposes of ritual. Adherents of Reconstructionist traditions may celebrate the four Gaelic festivals of Samhain, Imbolc, Beltane and Lughnasadh.
Some eclectic Neopagans, such as Wiccans, combine the Gaelic fire festivals with solstices and equinox celebrations derived from non-Celtic cultures to produce the modern, Wiccan Wheel of the Year. Some eclectic Neopagans are also influenced by Robert Graves's fictional "Celtic Tree Calendar", which has no foundation in historical calendars or actual ancient Celtic Astrology.
See also 
- Koch, John T. Celtic Culture: A Historical Encyclopedia. ABC-CLIO (2006). Page 330.
- Duval, P.M. and Pinault, G., Recueil des inscriptions gauloises, Tome 3: Les Calendriers (Coligny, Villards d'Heria), CNRS, Paris, 1986, pp. 35.
- Danaher, Kevin (1972) The Year in Ireland: Irish Calendar Customs Dublin: Mercier ISBN 1-85635-093-2; pp. 200–229
- McNeill, F. Marian (1961) The Silver Bough, Vol. 3. Glasgow: William MacLellan; p. 11-42
- Wade-Evans, Arthur. Welsh Medieval Laws. Oxford Univ., 1909. Accessed 31 Jan. 2013.
- Loth, Joseph. Les mots latins dans les langues brittoniques, E. Bouillon, 1892, p. 44 (et al.).
- Dictionary of the Irish language, Royal Irish Academy, 1983. Online
- Jackson, Kenneth Hurlstone. A historical phonology of Breton, Dublin Institute for Advanced Studies, 1967, p. 296, p. 248.
- Hamp, Eric. "The Indo-European Roots *bher- in the Light of Celtic and Albanian", in Zeitschrift für celtische Philologie, Volume 39, 1982, pp. 205-218.
- Bonewits, Isaac (2006) Bonewits's Essential Guide to Druidism. New York, Kensington Publishing Group ISBN 0-8065-2710-2. pp.134
- McColman, Carl (2003) Complete Idiot's Guide to Celtic Wisdom. Alpha Press ISBN 0-02-864417-4. pp.12, 51
- Hutton, Ronald (1991) The Pagan Religions of the Ancient British Isles: Their Nature and Legacy. Oxford, Blackwell ISBN 0-631-18946-7 p.337
- Hutton (1991) pp.145
Further reading 
- Brennan, Martin, 1994. The Stones of Time: Calendars, Sundials, and Stone Chambers of Ancient Ireland. Rochester, Vermont: Inner Traditions
- Brunaux, Jean-Louis, 1986 Les Gaulois: Sanctuaires et Rites Paris: Editions Errance
- Duval, Paul-Marie, et Pinault, Georges [eds] Recueil des Inscriptions Gauloises (R.I.G.), Vol. 3: The calendars of Coligny (73 fragments) and Villards d'Heria (8 fragments)
- Delamarre, Xavier, Dictionnaire de la langue gauloise, Editions Errance 2003.
- Dictionary of the Irish language, Royal Irish Academy, 1983. Online
- Geiriadur Prifysgol Cymru, University of Wales Press, 2nd Ed., 2002. Online
- Jackson, Kenneth Hurlstone. Language and history in early Britain, Edinburgh University press, 1953.
- Jackson, Kenneth Hurlstone. A historical phonology of Breton, Dublin Institute for Advanced Studies, 1967.
- Jenner, Henry. A Handbook of the Cornish Language, AMS Press, 1904, p. 203ff.
- Koch, John (ed.), "Calendar, Celtic", in Celtic Culture: a historical encyclopedia, ABC-CLIO, 2006, pp. 330–332.
- Lambert, Pierre-Yves, La langue gauloise, Editions Errance, Paris, 1995, pp. 109–115.
- Loth, Joseph. Les mots latins dans les langues brittoniques, E. Bouillon, 1892.
- Matasović, Ranko. Etymological Dictionary of Proto-Celtic. Brill Academic Publishers, 2009.
- Nance, Robert Morton. A Cornish-English dictionary, Federation of Old Cornwall Societies by Worden, 1955.
- Pokorny, Julius, Indogermanisches etymologisches Wörterbuch, Bern-Munchen 1959-1969.
- Schrijver, Peter. Studies in British Celtic Historical Phonology. Rodopi, 1995.
- Vendryes, J. Lexique étymologique de l'irlandais ancien. Dublin-Paris, 1959-(still in progress).