Everybody knows the secular holiday of Halloween. But not everybody knows it derives from a holy day, All Saints' Day on Nov. 1, which is followed by All Souls' day on Nov. 2.

The root word of Halloween - ''hallow'' - means ''holy.'' The suffix "een" is an abbreviation of "evening." It refers to the Eve of All Hallows, the night before the Christian holy day that honors saintly people of the past. "All Saints is a celebration of the communion of saints, those people we believe are in heaven, through good works and God's grace," said the Rev. Richard Donohoe, vicar of Catholic Charities for the Diocese of Birmingham.

All Souls' Day is a day to pray for all souls. Among Catholics, prayers are offered for those in purgatory, waiting to get into heaven. On All Souls' Day, Catholic churches have a Book of the Dead, in which parishioners have an opportunity to write the names of relatives to be remembered. "That's placed near the altar," Donohoe said. "That's done all through November. It's an All Souls' tradition."

Most Episcopal churches will observe All Saints' Day on Sunday, Nov. 3. Churches often read the names of those who have died in the last year.

More than a thousand years ago in Ireland and Britain, a common custom of Christians was to come together on the eve of the feast of All Hallows Day to ask for God's blessing and protection from evil in the world. Often, they would dress in costumes of saints or evil spirits and act out the battle between good and evil around bonfires. That's the source of the modern observance of Halloween.

The Christian concept of the importance of the individual soul underlies All Saints' Day and All Souls' Day, which are observed worldwide primarily in the Catholic and Anglican traditions. The ''Dia de los Muertos,'' or ''day of the dead,'' in Latin countries keeps alive some of the tradition of honoring souls of the dead. "All Hallows was considered a time when evil could manifest itself," Donohoe said. "We do believe in the visible and the invisible. There is good and there is evil. There is invisible evil and invisible good. It's an acknowledgement of that existence."

In the Catholic Church, Nov. 1 is a holy day of obligation, when all Catholics are expected to attend Mass. At St. Paul's Cathedral downtown, in addition to the morning and 12:10 p.m. Mass, there is a Mass set for 6:30 p.m. At St. Peter the Apostle Catholic Church in Hoover, there is a noon Mass and a 6:30 p.m. Mass. The All Souls' Mass on Saturday, Nov. 2, at St. Paul's Cathedral will be at 10 a.m.; and 9 a.m. at St. Peter.

"All Saints' Day and All Souls' Day are related, but they are two separate celebrations," Donohoe said. "On All Saints' Day there's a call to live as saints, to remind us how we're supposed to live. On All Souls' Day, we're talking about all souls and asking God's mercy for them. We're talking about those people who have died before us, and their process of getting to heaven, through Christ."

All Saints' Day emanates from early Christian celebrations of martyrs in the Eastern Church, Donohoe said. "It has its roots all the way back to the fourth century," he said.