The Celtic measurement of time appears to have assumed that darkness preceded light. Thus the Celtic calendar of pre-Christian times measured the year as beginning with the onset of winter. The Old Irish name for the first day of the new year is Samain, usually assumed to be 1 November in the Julian and Gregorian calendars (but 11 November in Gaelic Scotland). The beginning of the light half of the year was Beltaine, 1 May (or 15 May in Scotland). The dark half of the year was further divided by Imbolc, 1 or 2 February; and the light half of the year was divided by Lughnasa, 1 August (in Scotland sometimes as late as 29 September).
Key to our understanding of the Celtic measurement of time are the bronze tablets unearthed in 1897 at Coligny, 14 miles NNE of Bourg-en-Bresse (Ain) in eastern France, the most extensive document in the Gaulish language yet found (1st cent. ad) and now preserved at Lyons. They detail sixty-two consecutive months, approximately equal to five solar years. Months are thirty or twenty-nine days and are divided into halves. The lunar year of twelve months was adapted to the solar year by the intercalation of an extra month of thirty days every third year. Months are indicated either MAT [good or auspicious] or ANM [an abbreviation for anmat, not good]; remnants of this usage can be seen in the Welsh Triads which list certain events as mad [fortunate] or anfad [unfortunate].
See A. and B. Rees, ‘Light and Dark’, ch. 3 of Celtic Heritage (London, 1961, 1973), 83–94;Kevin Danaher, The Year in Ireland: Irish Calendar Customs (Dublin, 1977);Paul-Marie Duval, ‘Les Calendriers’, in Recueil des inscriptions gauloises, iii (Paris, 1985);Garrett Olmsted, The Gaulish Calendar (Bonn, 1992).