La Vigilia di Natale, also known as the Feast of Seven Fishes, is the Italian tradition of eating an extensive, seafoody dinner before Midnight Mass on Christmas Eve. Ok, not necessary the Italian tradition. It’s actually more common in the southern and Adriatic parts of Italy, celebrated in the regions between the island of Sicily and Rome. Once you get past Rome you’re likely to run into Italians who have never even heard of the seven fishes.
The feast goes by several different names. There’s La Vagilia di Natale (Vigil of the Nativity), Cenone (great supper), Cena della Vigilia di Natale (the supper of the Vigil of the Nativity), and Festa dei Sette Pesci (the Festival of Seven Fish). It includes all types of fish, including shellfish.