Like the Celts, the Europeans of that time also believed that the spirits of the dead would visit the earth on Halloween. They worried that evil spirits would cause problems or hurt them. So on that night people wore costumes that looked like ghosts or other evil creatures. They thought if they dressed like that, the spirits would think they were also dead and not harm them.
Many dressed up as souls of the dead and were understood to be protecting themselves from the spirits by impersonating them. More about that below.
Celts dressed up in white with blackened faces during the festival of Samhain to trick the evil spirits that they believed would be roaming the earth before All Saints Day on November 1st.