Halloween is all about the dead. While pagans and Christians have dramatically different notions about life, death and the afterlife, the two traditions that gave us Halloween agree on this: The dead are still with us. They must not be ignored.

Pagan rituals » The ancient pagan holiday Samhain (pronounced sow-en) had a big influence on how Halloween came to be celebrated in 20th-century America.

Samhain, which translates as "summer's end," was celebrated by the Celtic people of Ireland as the beginning of the dark half of the year, a new year and the greatest of four yearly festivals. It was associated with slaughtering livestock before winter, but its spiritual aspect was the most significant.

The Celts believed that there was a special quality to this time of year. At the border between two years, the veil separating this world from the next was especially thin, allowing spirits of the dead to roam freely.

While those sometimes caused trouble and damaged crops, Celts also thought they made it easier for Druids, or Celtic priests, to make predictions about the future. The prophecies were a source of comfort during the long winters.

Priests built big, sacred bonfires and sacrificed crops, animals and perhaps people to Celtic deities. The celebrants also lit their own hearth fires anew with embers from the blaze.

Samhain was a lot of fun for Celts, who dressed in costumes with animal heads and skins, pulled pranks on their neighbors and knocked on doors for food and treats.

There are some indications that Samhain originally was celebrated in November and that the festival migrated to Oct. 31 as pagan and Christian traditions braided together.

Christian roots » The name of the holiday, Halloween, reflects its Christian heritage.

It is a contraction of All Hallows' Eve, the eve of All Saints Day, which is the day early Christians honored holy people who had died. The earliest reference to a celebration of All Saints was in the fourth century after Christ, but it was established as a feast day in A.D. 609 by Pope Boniface.

All Saints Day was celebrated in the spring for several centuries, but Pope Gregory IV moved it to Nov. 1 in A.D. 835 (Orthodox churches still mark All Saints in the spring).

There is dispute about the reason. Some Catholic historians say it was moved so that pilgrims flooding into Rome for All Saints Day could be fed from the recent harvest. But most historians say All Saints Day was shifted to Nov. 1 to help Christian missionaries in Ireland. They wanted to replace the great pagan feast of Samhain with a Christian feast, and All Saints Day fit the ticket because it, too, addressed the connection between the living and the dead.

Several centuries later, the Roman Catholic Church added another celebration -- All Souls Day -- on Nov. 2. The memorial first was observed in A.D. 998. The church officially adopted the feast in the 14th century. Many All Souls traditions became precursors to Halloween, including leaving food for the dead, costume parades, decorating graves and starting bonfires to light the return of souls.

Pagans today » Wiccans, Druids and a variety of other modern Pagans celebrate Samhain, the beginning of a new year, in a variety of ways on Oct. 31. As in most U.S. families, pagan children dress in costumes and trick or treat. Either on Oct. 31 or another night, families gather to play games, to communicate with the dead and to honor those who have died in the past year.

Divination is popular to help pagans gain insight about their pasts and futures. Many leave a plate of food outside their homes Oct. 31 so the souls of the dead will have something to eat. They also place candles in the windows to guide spirits and bury apples or other fruit to feed the dead.

Christians today » For Christians, Halloween is the vigil of two days in which they remember the connectedness of the living and the dead through what is known as "the communion of the saints."

Nov. 1 is All Saints Day. Set aside as the day to honor all holy people who have lived and to ask their prayers, All Saints is a holy day of obligation in the Roman Catholic Church. That means Catholics are required to attend Mass.

Nov. 2 is All Souls Day, when Catholics pray for souls in purgatory. The belief is that souls bound for heaven but not yet perfect enough to be in God's presence can be purified through the prayers of the living.

Some other Christian denominations also observe the two feast days, but most do not.

Throughout the world, various customs have arisen to commemorate the feast days. In Mexico, for instance, Catholics celebrate Días de los Muertos (Days of the Dead) for three days, picnicking on ancestors' graves and eating tiny candy skeletons. Children who have died and adults who have died are remembered in turn.

Attacks on Halloween » Some Christians who believe Halloween is a pagan holiday that invites or celebrates evil have been trying to shut it down for centuries. In Post-Reformation England, All Saints Day and its vigil, All Hallows Eve, were suppressed. The distinctly Catholic feast days also were outlawed -- as was Christmas -- by the Puritans in America. Large numbers of Irish immigrants in the 19th and 20th centuries, however, helped popularize the holiday in America.

Although there have been efforts to sanitize Halloween of its theme of death by substituting harvest festivals and carnivals, those largely have not succeeded.

More Pagans » Pagan religions such as Wicca are drawing increasing numbers of adherents in the United States. According to the American Religious Identification Survey, the number of Wiccans -- witches and warlocks -- more than doubled from 2001 to 2008, from 134,000 to 342,000.

Neo-pagans are people who follow any one of a variety of pre-Christian religions that usually blend old and new traditions. Many follow an earth-based or nature religion.

Sources » "The Halloween Encyclopedia," "Religious Holidays and Calendars" 2nd edition, "The Real Origins of Halloween," American Religious Identification Survey, history.com