I always do a big Christmas Eve dinner, with a Polish slant. We long ago abandoned the old tradition of fish and cold food, but keep a few of the stuff we all liked - at lest some of us. Herring was always a part of the Christmas eve meal, and I could usually find the kind I like, simple salt cured non flavored herring. Historically it was headless mixed or milkers. My dad would soak them overnight in fresh water to get the salt out them skin them and cut them into chunks and put them in a vinegar solution or for my mother oil. We always put sliced onions on top of both. Been part of our holiday for generations.
The last few years I've been able to get herring fillets, not quite the same as the are boneless, but prep is very quick. It was the third year for the 4kg tub I bough and wanted to get a fresh tub this year, went to the fish market and no luck. Looked a bit on line and found not much available there either. As they are in a lot of salt the 1/3 jar I had left were still good, just did not look real appetizing in their original state, but once washed up and soaked were fine.
I'll add salted herring to the list of thing that are difficult to find around here, as is the Chicago Almond poppy filing I'm on the hunt for. Next time I'm in Chicago close to the holidays I will have to do some searching. Odds are more favorable there I'd guess.