The phrase "it is customary (for somebody) to do something" means that it is a common practice, tradition, or expectation for a particular person or group to behave in a certain way or perform a particular action in a given situation.
It is customary for newlyweds to cut the wedding cake together.
It is customary for guests to bring a gift to a housewarming party.
In these examples, the phrase is used to describe social norms, traditions, or expectations that are widely accepted or followed within a particular group or community.
The phrase can also be used to describe personal habits or preferences that someone consistently follows. For example:
It is customary for her to take a walk after dinner every evening.
It is customary for him to wear a suit and tie to work every day.
In these cases, the phrase is used to describe individual behaviors that someone regularly exhibits, often as a matter of personal preference or habit.