The cook's responsibility is to serve seven different kinds of fish. According to one source (Gomestic.com), the origin of the "seven" is unclear. Perhaps it's because of the seven days it took God to create the world, and others see a connection to the Seven Hills of Rome. Still others suggest a relationship with the seven sacraments, or even the seven sins of Catholocism.
ItalianFoodForever cites a medieval Catholic tradition of abstinence as part of the origin for the practice. Eating seafood meant refraining from consumption of meat or dairy products on Fridays and other occasions.
At least one blogger is dedicated to the subject: http://www.sevenfishesblog.com/, and he offers the photo linked to here. The Wikipedia entry for this tradition offers yet another theory: . . .
that seven is a number representing perfection: the traditional Biblical number for divinity is three, and for Earth is four, and the combination of these numbers, seven, represents God on Earth, or Jesus Christ.
Some comments on numerology in a later post.