A CUSTOM – a traditional and widely accepted way of behaving or doing something that is specific to a particular society, place, or time.
A COSTUME – a set of clothes: 1) typical of a particular country or historical period ; 2) worn by an actor or performer for a particular role.
Be careful how you write and say these two different words:
A CUSTOM /ˈkʌstəm/
A COSTUME /ˈkɒstjuːm/