Much confusion reigns regarding the origin of the name of the day called Samhain (traditionally November 1st) in Irish.
The Irish name Samhain comes from the Old Irish Samain, which in turn would have developed from Proto-Celtic *Samanis, which in turn reflects a possible Proto-Indo-European *Som-h2nis, this being an o-grade derivative of *sem– ‘single’. Germanic cognates of Proto-Celtic *Samanis include Gothic samana, Old Norse saman, Dutch samen and German zusammen, all derived from Proto-Germanic *samana and all meaning “together”. A further cognate is Sanskrit samana which also means “together”.
It is rather clear to me that the word Samain originally had a meaning “together” and denoted a gathering. Indeed, the main feature of Samain is the gathering together of the living with their departed ancestors. It is also rather clear to me that the word Samain has nothing at all to do with the Gaulish month name Samonios.
It is usual to see Old Irish Samain incorrectly derived from the term Samonios which is found on the Celtic Coligny Calender (see for example https://en.wiktionary.org/wiki/Samain ). The big problem with this is that Samonios would have given *Samuine in Old Irish. Although the forms Samuin and Samfuin are found in Old (or Middle) Irish texts, these are most likely due to a false interpretation of the word’s origin by medieval Irish monks. In any case, there certainly are no instances whatsoever of an –e at the end of the word.
The Celtic month name Samonios is clearly derived from the Proto-Celtic word for summer, this being *samo– from an original *sṃh2o-. This interpretation of Samonios is undoubted due to the existence of another Celtic month called Giamonios six months away from Samonios, this other month name being clearly derived from the Proto-Celtic word for winter: *giamo– The Germanic words for summer (such as English summer) are all related to Proto-Celtic *samo-, these all coming from Proto-Germanic *sumaraz, which developed from an original *sṃh2o-ros.
By the way, there are some who would interpret Samonios and Giamonios as meaning “end of summer” and “end of winter”, as if the –ni– element meant “end”, this ostensibly being derived from the Proto-Indo-European preposition *ni meaning “down”. Much more likely is that the two month names have n-stem forms of the words for “summer” and “winter” (*samon– and *giamon-) followed by the common relative suffix –ios meaning “the one of/relating to”.
But interpreting Samonios as “summer’s end” allows people to (mis-)associate the month Samonios with the Irish Samain, making Samonios an October/November month (and Giamonios an April/May month). This interpretation causes the month Ogronios (four months after Samonios) to be a February/March month, which suits the usual interpretation of Ogronios as the “cold month”.
One problem with this is that the word for “cold” in Proto-Celtic (yielding Old Irish úar and Welsh oer) is usually given as *ougros, not *ogros (but then, the Coligny Calender contains numerous instances of a serious incapacity to get orthographies straight…). Moreover, I don’t even believe that *ougros was the original word for “cold” in Proto-Celtic, but rather *usaros (see the commentary to the entry for uar in my Vindonian wordlist at https://vellaunos.ca/2021/04/18/vindonian-wordlist/). As for Ogronios, I see a clear connection to Proto-Germanic *akraną (which came from *ogronom and which gave English acorn) and to the Celtoid word *agrenios meaning “plum” which became eirin in Welsh and airne in Irish, further connections being Lithuanian uoga “berry” and Russian jagoda “berry”.
One more thing to address: the existence of the terms Trinvxsamo and Trinosam on the Coligny Calender. These terms have been taken to be abbreviations of *Tri-noxtion-samoni, which is interpreted as the “Three Nights of Samain”, this corresponding quite nicely with the Irish festivity. One important problem is that the first term has –nux– rather than –nox-, but that could just be an orthographic problem, especially since the second term does have –nos– (again, we are dealing with a very substandard orthography…). A much bigger problem is that the term Trinvxsamo only appears in the month called Samonios. As mentioned above, the name of this month literally means “the one of summer” and it most probably occurred in what we call June and July – nowhere near the beginning of November.
In any case, I am rather partial to interpreting the term Trinvxsamo as *Trin-ouxsamo– meaning “Triply Most-High”. This would be a Celtic equivalent of the Roman Ter Maximus “Thrice Great”, an epithet of Mercury, which in turn is a calque of Greek Trismegistos “Thrice Great”, an epithet of Hermes. My opinion is that the Celtic God Lugus may well have been identified as Trinouxsamo “Triply Most-High” (cf. the Irish epithets Ildanách “Many-Skilled” and Samildanách “Equally Many-Skilled” for the Irish God Lugh).
Having said all this, I wouldn’t say that there was no equivalent to the Irish Samain among the Celts. But I doubt it. I will also say that I honestly believe that people who try to find connections between the Celts and the Irish often end up promoting things that simply aren’t true.