Here is an explanation from Wikipedia:
Italians around the world celebrate
Christmas Eve with a Feast of the Seven Fishes, also known as La Vigilia.
It is a meal that consists of, typically, seven different
seafood dishes. Some Italian families have been known to celebrate with 9, 11 or 13 different seafood dishes. This celebration is a commemoration of the wait,
Vigilia di Natale, for the midnight birth of the baby
Jesus.
There are many hypotheses for what the number "7" relates to, one being the number of Sacraments in the
Catholic Church.
In 2004 writer
Robert Tinnell launched a webcomic, a romantic comedy set against the background of the traditional feast as observed in 1983. Within months the comic became very popular, eventually resulting in a printed compilation. That book, Feast of the Seven Fishes, the Complete Comic Strip and Italian Holiday Cookbook, garnered a 2006 nomination for Best Graphic Album – Reprint. The book is now in its second printing and scheduled to be adapted as a film in January 2008.
The components to the meal are similar for most families as there are always some seven combinations of:
anchovies,
sardines, dried salt
cod, smelts,
eels,
squid,
octopus,
shrimp,
mussels,
oysters, and
clams. In the mix are
pastas,
vegetables, baked or fried kale patties, baked goods, and the pride-filled homemade
wine.
Tradition in my family has been that my wife and I host Christmas Eve and Day. Christmas Eve is usually a different set of food every year. We had the Feast of the 7 fishes one year. This year we are going meatless in honor of my FIL side of the family. Christmas Day we always have Prime Rib.