Nonetheless, Samhain may have involved human sacrifice, although this is not very clear one way or the other.
Gaius Julius Caesar said the Britons engaged in human sacrifice. Who are we to question the Imperator of Gaul and Cleopatra?
We are those who have a) no vested interest to smack down the barbarians and annex their lands, and b) archaeological research at our disposal.
Samhain did not involve human sacrifices. It did involve rather large-scale slaughter of cattle, though. It's called 'culling the herd' and its objective is to maximise the chances of survival of both herds and humans through the winter.
On the other hand, Britons, very much like Romans, practised augury. If they felt the need to divine by examining human bodies, rather than animals, they must have been in dire straits. Like, oh, I don't know - having a foreign army invading their lands, perhaps?