Halloween

Wikipedia's article on Halloween.

The name applied to the evening of October 31, which precedes the Christian feast of All Saints' Day. Halloween traditions are thought to have originated among ancient Druids, who believed that evil spirits were active on that evening. Ancient Celts observed Halloween as the last evening of the year and believed that during it the spirits of the dead revisited their earthly homes. After the Romans conquered ancient Britain about 55 BC, they added features of a Roman harvest festival to Halloween. The concept of ghosts and witches is still common to all Halloween observances.