The reason behind the Carnivale is that it is the last day before the fast begins.  Lent is meant to symbolize the fast of Jesus during his 40 days in the desert. While the origins may date back to apostolic times, it was formalized by the first council of Nicea in 325 ce. However there is much scholarly debate as to the timing of the fast throughout the ages and by the different schisms of the Catholic Church. The Eastern Orthodox start and end their lent on different dates than the Roman Catholics who traditionally started their's on “Ash Wednesday” and ending on Easter Day.

Throughout history the nature of what constitutes a “fast” also evolved. Originally, one meal a day was allowed in the evening, and meat, fish, eggs, and butter were forbidden. Gradually the restriction was an abstention from “food derived from the flesh”.  This not only meant meat, but also dairy products such as eggs.  This may explain the tradition of gifting eggs at Easter.  

A common fallacy is that fish was eventually “allowed” by the Church to prop up the Italian fishing industries. There is no evidence to support the claim. Italy, despite being a peninsula, does not really enjoy a robust fishing industry. The sea currents make fishing much more seasonal, and fresh fish barely finds it way inland. Keep in mind “Italy” wasn’t a unified country until 1860.  The reason was more likely due to the role of fishes and fishermen played in the scriptures of Jesus.

Given that most of Italy was still in winter during the time vegetable choices were also skimpy.  So the prohibition of meat, dairy and the scarcity of vegetables suited the penance sought by Catholics during their holiest period. Today many Catholics in Italy will forgo meat on Friday in honor of this tradition. Of course today that is not so much of a burden with many vegetarian dishes available .  You'll find many hearty soups such as the classic "Pasta e fagioli", a favorite soup this time of year.