Samhain is genuinely attested in early Irish literature, which puts it in a better evidential position than most of the modern Wheel of the Year. What is attested is narrower than what is usually claimed.
The medieval Irish sources, the tale literature, the law tracts, and glossaries such as Cormac's Glossary, treat Samhain as one of four quarter days, with Imbolc, Beltane and Lughnasadh, and place it at the beginning of winter. The tales set major assemblies at it, notably the gathering at Tara, and a number of otherworldly incursions in the sagas happen at Samhain, which is the basis of the thin-veil claim.
Ronald Hutton, whose survey of the British ritual year is the standard scholarly treatment, argues that the earliest sources present it primarily as a harvest and herding festival, the point at which livestock were brought in from summer pasture and stock decisions made, and that a specific ritual connection with the dead is not evidenced in them. The otherworld episodes in the sagas are literary, occurring at a date that marks a boundary, and the sagas were written down by Christian monks centuries after conversion.
There is no evidence for druidic ritual at Samhain, none for human sacrifice, and none for a god of the dead named Samhain, a figure invented in nineteenth-century writing from a misreading, and repeated since in anti-Halloween tracts.
The name means, plausibly, summer's end. The suggestion that it derives from a deity's name is not accepted by Celticists.