The tradition of dressing in costume for Halloween has both European and Celtic roots. Hundreds of years ago, winter was an uncertain and frightening time. Food supplies often ran low and, for the many people afraid of the dark, the short days of winter were full of constant worry. On Halloween, when it was believed that ghosts came back to the earthly world, people thought that they would encounter ghosts if they left their homes. To avoid being recognized by these ghosts, people would wear masks when they left their homes after dark so that the ghosts would mistake them for fellow spirits.