We don't have synonyms for samhain yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are samain and samhan.
(n) :
A holiday, falling on the night of 31 October to 1 November, celebrated by the ancient Celts and by modern neo-pagans as the beginning of winter and the new year, and a time during which the spirits of the dead could return to the earth.
(n) :
A holiday, falling on the night of 31 October to 1 November, celebrated by the ancient Celts and by modern neo-pagans as the beginning of winter and the new year, and a time during which the spirits of the dead could return to the earth.