This is really interesting.

My understanding is that Christianity also adopted Samheim as the night of the dead and renamed it Allhallows/ feast of all saints. Halloween is Hallow's e'en or evening, the night before All Hallows. All Saints Day and All Souls Day are times to honour the dead, pray for their souls etc. Different versions of Christianity and different nationalities have their own ways of observing these days. It was/is believed that the souls of the dead can reach earth at midnight on All Saints Day. Hence, the tradition of fearing/observing ghosts etc. which is how halloween started.