There are two main hypothesis regarding the etymology for Samhain.

The Old Irish samain, samuin, samfuin, refers to 1 November (lathe na samna, "samhain day"). Its meaning is glossed as "summer's end", and the frequent spelling with f suggests analysis by popular etymology as sam "summer" and fuin "sunset, end". Old Irish sam, c saminos derived from *samo- "summer".  Its cognates are Welsh haf, Breton hañv, Old Norse language sumar all meaning "summer", and Sanskrit sáma "season".

Confusingly, Gaulish Samonios (October/November lunation) corresponds to GIAMONIOS, the seventh month (the April/May lunation) and the beginning of the summer season.

In medieval Ireland, samain (oenaig na samna, "samhain night") was also the name of the principal festival, celebrated with a great assembly at the royal court in Tara.

W. Stokes in KZ 40:245 (1907) suggests an etymology from Proto-Celtic *samani with a meaning "assembly", cognate to Sanskrit sámana, Gothic samana . J. Vendryes in Lexique Étymologique de l'Irlandais Ancien (1959) concludes that these words containing *semo- "summer" are unrelated to samain, remarking that furthermore the Celtic "end of summer" was in July, not November, as evidenced by Welsh gorffennaf "July".