Since much of the history of Halloween wasn't written down for centuries; some of it is still sketchy and subject to debate. But the most plausible theory is that Halloween originated in the British Isles out of the Pagan Celtic celebration of Samhain. It goes back as far as 5 B.C. It was believed that spirits rose from the dead and mingled with the living on this day. The Celts left food at their doors to encourage good spirits and wore masks to scare off the bad ones. Some historians believe that the Romans who invaded England added a few of their own traditions to the celebration of Samhain; such as celebrating the end of the harvest and honoring the dead; others say that since the Romans never conquered the Celts (Ireland and Scotland) there was no mingling of cultures, and that the Celts celebrated the end of the harvest and honored their dead in this way, anyway!