Wikipedia

Christmas carp (German: Weihnachtskarpfen) is a traditional dish for Christmas Eve in Central Europe. In the regions where it is eaten, beliefs link its head to Christ's torture instruments and protection against witches. The fish is traditionally kept in a bathtub before preparation to remove muddy flavours and is commonly fried or served in dishes like gefilte fish. In some regions, leftover bones are placed in fruit trees for spring growth and its scales are carried for good luck.

I woulda guess England. Dunno why.

https://en.wikipedia.org/wiki/Common_carp

In Central Europe, it is a traditional part of a Christmas Eve dinner.

In the United States, carp is mostly ignored as a food fish. Almost all U.S. shoppers bypass carp, due to a preference for filleted fish as opposed to cooking whole. Carp have smaller intramuscular bones called y-bones, which makes them a whole fish species for cooking.

In Europe, common carp contributed 1.8% (0.17 Mt) of the total inland fisheries production (9.42 Mt) during 2015–2016. It is a major farmed species in European freshwater aquaculture with production localized in central and eastern European countries. The Russian Federation (0.06 Mt) followed by Poland (0.02 Mt), Czech Republic (0.02 Mt), Hungary (0.01 Mt) and Ukraine (0.01 Mt) represents about 70% of carp production in Europe during 2016.

In Utah, the common carp's population in Utah Lake is expected to be reduced by 75 percent by using nets to catch millions of them, and either giving them to people who will eat them or processing them into fertilizer.

Heh.

Europe:

"This is an excellent fish that we farm reserved for holidays."

US:

"You want free fish? You can have all you want."

"Does it have bones that I have to eat around?"

"Uh. Some."

"Not interested."

Here is Carpe Diem, a very cute short film featuring this tradition.

Official Vimeo mirror (somewhat region-locked): https://vimeo.com/channels/animationaub19/341755071

There are references to Cosy Dens, hallmark Czech Christmas movie set during the 1968 Prague Spring and Soviet invasion, as well as graduation films by their classmates.