How did Halloween get its name? From All Saints Day, which falls on today (Nov. 1). The original name was All Hallows Day, which means pretty much the same. (Despite what you may have heard from Harry Potter, hallows are holy persons.) All Saints Day is shortened from the official name, the Solemnity of All Saints and also called All Hallows or Hallowmas.
The root of the observance came from martyrdom, especially in the first five centuries of the Christian era. Churches began honoring members who were killed for their faith, saying Eucharist at their graves on the anniversaries of their deaths; but the task became harder as more died. So by the fourth century, they established one day to honor them all.
The holiday took its present form in the eighth century, when Pope Gregory III declared Nov. 1 as the day to remember the apostles, saints and martyrs. The day was picked to supplant Samhain, a Celtic festival for the end of summer, when the dead returned to visit. Many pagans reacted by simply moving their observance to the previous night. Hence the name All Hallows Evening, or Halloween.
All Saints Day is observed not only in Roman Catholic circles but also Anglican, Lutheran, Methodist and Wesleyan churches. Whether they see deceased members as especially holy or not, believers emphasize a spiritual bond between Christians in this world and the next.
— James D. Davis