Halloween is not exactly a typical holiday. Other holidays, like Christmas and Shavuot, celebrate an event. Halloween celebrates a lot of things, including the lives of people who aren't with us anymore.
The history of Halloween is not entirely a clear one. Here's how (we think) it started:
Many hundreds of years ago, a people called the Celts lived in Europe and on the British Isles. The Celts believed that the souls of the dead visited Earth on the last day of October. They had a festival in honor of these souls of the dead, and they called it Samhain.
In time, the Roman Empire conquered the Celts and took over some of their beliefs as well. This included Samhain. The Romans combined it with their own festivals. And since the Roman Empire spread across a great part of the known world, the idea that the souls of the dead visited Earth on the last day of October spread far and wide.
Many ideas from the Roman days still survive in the United States and in other Western countries. Halloween is one of them.
But how did we get the name Halloween?
In the 8th Century, the Catholic Church declared November 1 to be All Saints' Day. The church calendar had a number of days honoring saints already. November 1 was picked to be the day to honor all saints who didn't already have a day named in their honor.
And the mass that the Catholic Church celebrated on November 1 was called Allhallowmas. This meant "mass of all the hallowed [saintly people.]" It was commonly called "All Hallows' Day."
And somewhere along the line, the night before became known as Allhallowe'en, which was short for "evening before All Hallows' Day." It was then shortened to what we now call it, Halloween.
One last question: Why do people dress up as ghosts, goblins, vampires, and other scary creatures? The people who started all this Halloween business many years ago believed that if they appeared scary, they would scare away the spirits of the dead who were roaming the earth on All Hallows' Eve. These people also carried food to the edge of town and left it there, hoping the spirits would eat that food and not come raid the village.