Polish custom says that the Christmas Eve supper should feature fish - or at least eschew red meat. This is broadly part of the Christian custom of abstention in the lead up to Christmas Day.
Italy has a similar custom. The "Feast of the Seven Fishes" is an Italian tradition called "La Vigilia" - a very similar Latin-based name to the Polish "Wigilia".
However, Poland has developed its own twist to this broadly Christian tradition, with pride of place on the table taken by the "king" carp.
And this tradition appears to go back no further than the late 1940s when the communist Minister of the Economy Hilary Minc declared that "every Polish table shall have carp." It was a slogan, a declaration of intent:
"The Christmas-Eve carp is a new tradition, the result of World War II. Before the war it was just one among several fish eaten - like herring, pike and perch. After the war, Poland lay in ruins, so the authorities promoted carp because of the relatively low cost of carp farms...":
The website ciekawostkihistoryczne.pl investigates a little further and finds no carp at a diplomatic Christmas Even event from 1926 - let alone the 10 carp dishes customary today.
Sources: ciekawostkihistoryczne.pl, X
pt