Halloween began as the festival of Samhain. It was part of the ancient Celtic religion in Britain and other parts of Europe.
At the end of summer, the Celts thought the barrier between our world and the world of ghosts and spirits got really thin.
This meant weird creatures with strange powers could wander about on Earth.
The Celts had a big party. It was all about scaring away the ghosts and spirits.
Later, with the Christian religion, the day became known as All Hallows' Eve - the day before All Saints' Day on 1 November.