Halloween is one of the world's oldest holidays, dating back to pagan times. But it is celebrated today by more people in more countries than ever before. There's a simple reason: it is fun and it is good, clean, harmless fun for young and old alike! Also see Halloween around the world and see this page of current Halloween facts and statistics.
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!
Many centuries later, the Roman Catholic church, in an attempt to do away with pagan holidays, such as Halloween (and Christmas, which had been the Roman pagan holiday of Saturnalia) established November 1st as All Saint's Day (in French, la Toussaint), in celebration of all the saints who do not have their own holy day. This attempt to detract attention from the pagan celebration of Samhain didn't work. The celebrations on the eve of All Saint's Day continued to grow and change! During the massive Irish immigration into America in the 1840s, Halloween found its way to the United States, where it continued to flourish!
It is also believed that the Christian practice of celebrating the evening before a holiday, such as Christmas Eve, New Year's Eve, etc. came from the Jewish traditions. Jewish days and holidays begin with the evening before. Always have, as Judaism follows a lunar calendar in which the sunsets begin the new day. Many Christian groups now observe holy days from sundown on one day until sundown on the following day.
The modern name, Halloween comes from "All Hallows' Evening," or in their slang "All Hallow's Even", the eve of All Hallows' Day. "Hallow" is an Old English word for "holy person," and All Hallows' Day is just another name for All Saints' Day, eventually, it became abbreviated to "Hallowe'en" and then "Halloween."