It is widely believed that many Halloween traditions
originated in Britain
and Ireland from
Celtic harvest festivals which may have pagan roots, particularly the
Gaelic festival Samhain, and that this festival was Christianized as
Halloween. Samhain was the Celtic end of the harvest and the
beginning of winter. It is a time when the boundary with the
Otherworld thinned and the dead could appear in our world.
Costumes were worn to hide from the dead souls.
One of the more traditional costumes is of a witch
Another typical costume is the devil
Halloween in the US always includes pumpkins
Now, on to the rest of the costumes -- vampires, zombies, fairies, heroines, etc.