Custom

A way of behaving or a belief that has been around for a long time

EX: It is a Vietnamese custom to eat moon cake during the Mid Autumn Festival

Merriam-Webster Online Dictionary
custom (noun)
1.
a) a usage or practice common to many or to a particular place or class or habitual with an individual
b) long-established practice considered as unwritten law
c) repeated practice
d) the whole body of usages, practices, or conventions that regulate social life
2.
a) duties, tolls, or imposts imposed by the sovereign law of a country on imports or exports
b) the agency, establishment, or procedure for collecting such - customs
3.
a) business patronage
b) usu. habitual patrons - customers habit
custom (adjective)
1.
made or performed according to personal order
2.
specializing in custom work or operation - a custom tailor