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.
Nicholas Rogers, a historian at York University says that when people prayed for the dead at Hallow Mass, they dressed up. When praying for fertile marriages, the boy choristers in the churches dressed up as virgins. So there was a certain degree of cross dressing in the actual ceremony of All Hallows Eve
The carving of pumpkins originates from the Samhain festival, when Gaels would carve turnips to ward off spirits and stop fairies from settling in houses.