Clam Chowder is often served on Fridays because the restaurants wants to get rid of the seafood ordered from the previous weekend or early in the week.
The reason Clam Chowder is served on Fridays is because of a tradition of where Catholics were supposed to abstain from meat on Friday.
So it just kept being a tradition in serving the clam chowder on Fridays.
Clam chowder is often served in restaurants on Fridays in order to provide a seafood option for those who abstain from meat every Friday, which used to be a requirement for Catholics before liturgical changes in Vatican II
You can get clam chowder at any time but sometimes clam chowder becomes a special on Friday and some other days as a way to get rid of the seafood that was ordered so it does not go bad.