What is the Feast of the Seven Fishes? According to Mario Batali, "It's what Italians do when they say they're fasting." More precisely, the Feast is a meal served in Italian households on La Vigilia (Christmas Eve). In many parts of Italy, the night is traditionally a partial fast, during which no meat should be served. 

But in true Italian style, this proscription has morphed into something very unfastlike indeed: course after course of luxurious seafood dishes, often as many as 7, 10, or even 13. "No one's quite sure of the significance of the number," says Batali. "Some families do seven for the sacraments. Some do ten for the stations of the cross. And some even do 13 for the 12 apostles plus Jesus."

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Source: http://www.epicurious.com/