I am trouble by the eymology of Sanskrit sámana. Looking up samana I find an alternative origin from the root word sama "summer".
"Samana appears to have been a seasonal festival at the beginning of 'sama' or the summer season. According to ancient custom Indrani was worshipped by women at the 'samanas'. ..."
Bulletin of Ancient Indian History and Archaeology, Issue 1, Department of Ancient Indian History, Culture and Archaeology, University of Saugar., 1967
Vedic Index of Names and Subjects, by Arthur Berriedale Keith, 1995, says:
"Sama appears originally to have denoted 'summer,' a sense which may be seen in a few passages of the Atharavaveda [citation]. Hence it also denotes more generally 'season,' a rare use [citation]. More commonly it simply 'year' [citation]; but in one place the Satapatha Brahmana [citation] interprets it in teh the Vajasaneyi samhita [citation] as meaning 'month,' a doubtful sense."
Gaulish Samonios is a month, also the start of the year and the summer season. The is true for the Irish, for [Cet] Samain starts the year and the season of summer.
Keith continues: "Samana is a word of somewhat doubtful sense in the Rigveda. Roth [citation] renders it either 'battle' [citation] or 'festival'. [citation]. Pischel [citation] thinks that it was a general popular festivity to whih women wen to enjoy themselves [citation], poets to win fame [citation], bowmen to gain prizes at archery [citation], horses to run races [citation]; and which lasted until morning [citation or until a conflagration, caused the fires kept burning all night, scattered the celebrators [citation]. Young women [citation, elderly women [citation], sought there to find a husband, and courtezans to make a profit of the occasion [citation]."
I interpret the festival of Samain as containing a mock battle (summers victory over winter) and a time for marriages, much like the above description of Samana.
While I find the brythonic cognate of samain to be heven/hefen:
Geiriadur Prifysgol Cymru define Hefin as:
"Haf, tymor neu amser haf; yn perthyn i’r haf neu’n nodweddiadol ohono, hafaidd, tesog:
summer, summer-time; pertaining to or characteristic of summer, aestival, hot, sunny."
and realted word Hafin, as:
"Hin hafaidd, hinon haf, heulwen:
fine or summer weather, the serene season of summer, sunshine."