Christians have a custom of putting up & decorating a tree at Christmas.
Get the idea? When you say "it's customary" you are referring to the practises of a certain people.
In Canada, there is the general belief that the British always have tea at 4:00, as a matter of their customs. From what I've read here in answers, I know that isn't true for all British, but are you picking up what I'm putting down?
