Samhain was as time to commune with the dead. This is exactly the same thing that Roman Catholics due on All Souls' Day. On this day the Catholic Church has services and offers prayers for the dead. In reality prayers were not only prayed for the dead, they were prayed to the dead. Only now the dead ones prayed to were called saints.
Necromancy is the communication with the dead.
It is the calling upon the spirits of the dead for the purposes of revealing the future or influencing the course of events. This is exactly what Roman Catholics do when they pray to dead Christians. In Irish folklore the carved pumpkin symbolized a damned soul named Jack. According to the story, Jack was not allowed into heaven or hell, but was condemned to roam the darkness with his lantern until Judgment Day. Many ex-witches and satanists have come out against the celebration of Halloween by Christians.