The Feast of the Seven Fishes has become a Christmas Eve tradition for Italian-Americans, based on the Catholic practice of refraining from eating meat on that night.
Why seven? There's no definitive answer. In fact, not everyone limits themselves to that number. Some feasts will feature 10, 12 or 13 dishes. Aren't you glad we cleared all that up?
Source: http://www.washingtonpost.com