It is customary (for somebody) to do something

something that is customary is normal because it is the way something is usually done (= usual)

In some cultures it is customary for the bride to wear white.

Gourmets advance a similar argument every year at Christmas when it’s customary to load the table with tasty crustaceans, and Der Spiegel (the print sister magazine to Spiegel Online) has reported on this, as well.