Trinox Samoni, found on the Coligny Calendar

The right hand side of the image above shows the fifteen days that comprise the second half of the month Samon. The heading 'Atenoux' is found before every second half month, and means 'returning dark' - it begins on the evening of the last quarter moon (since the months begin on the evening of the first quarter moon).
On day 17 of this particular Samon (there are five years recoded on the insciption) is seen Trinox Samoni, which has
often been confused with Samhain.

A great deal of confusion has arisen about the reference on the 'Coligny calendar', a bronze engraving from 1st century BC Gaul, to an event recorded in the month of Samon, the first of this genuinely Celtic year, termed the 'Trinox Samoni'. Very often, in both academic and New Age publications, this term has been incorrectly translated to "The three nights of Samhain", on the basis of phonetic correspondences with the Old Irish 'trenae samhna'; and the month Samon has been equated with the Irish month of November, Samhain. An example of this is the case of Tadhg MacCrossan in his book "The Sacred Cauldron" (Llewellyn 1991, pgs. 207-208):

"Samhain is cognate with a Gaulish month name, Samonios, which is Samon, "summer", with the derivational -ios ending. Samhain or Samonios means literally "summery". But why would the end of the summer half of the year start with a month named "summery one"? The answer is found in the etymology, for the PIE root sem-, "summer", means the time of year for se-, "sow" or the sowing. Summer was originally the name for the half of the year that was spent in sowing, and thus the last month was its culmination and ending or fulfilment. Irish glossers would later suggest a compound, "Sam-fuin", "summers end", since the festival marked its ending, but the phonetic correspondence between the Gaulish phrase "trinouxtion samoni" and the Old Irish " trenae samhna" shows that both mean the same "three nights of Samhain".

Another example is found in a serious astro-archaeological study concerning the timing of the Celtic fire feasts with the heliacal risings of important stars, which nevertheless remains a very interesting consideration, is found at www.brera.mi.astro.it/~gaspani/celtcab.txt.

The month in the 'Coligny calendar' that equates to November is in fact Giammon, which means 'Winter'. This corresponds to the older Irish name for the month of November, Gamain, recorded in Cormac's glossary, a point made by PW Joyce in 1903. Gamain is the exact Irish equivalent of Gaulish Giamonios, the seventh month of the Celtic Calendar. The name of the first month, Samon, means 'Summer', also found in the ancient Irish name for May, cet-Samhin. The older Irish 'samos', meaning summer, is indicated by Henri Herbert in 1934; thus the Samon festival Trinox Samoni, so often equated with Samhain, is in fact a festival relating to the Celtic summer.

The fact that Samhain comprised a three day holiday prior to its adoption into the Christian feasts of Hallowe'en and All Saints' Day was mistakenly identified at the end of the nineteenth century with the apparent three-day feast that occurs in the month of Samon, namely "Trinvx Samonos": The contention therefore that Samon=Samhain is very understandable. It has been shown, in the light of a further century's research, however, to be unfounded.