Where I grew up in the US, Fat Tuesday is observed by the consumption of donuts, or the not-as-sweet fastnachts if you're traditional.
The Swedish tradition (Fett Tisdag) uses the particularly Swedish semlor, which are massive cream-filled, sightly spicy (Wikipedia says it's cardamom) pastries.
The idea behind every Fat Tuesday tradition is to load up on as much delicious food as you can before the restrictive fasting of Lent, and that's the same idea with the above semlor. But there's another pre-fasting food tradition in Sweden I didn't learn about until recently.
In the days of yore, after Christianity finally spread to Sweden (it took longer here than in the rest of Europe), Friday was established as a Catholic day of fasting. Thursday, then, was a day to eat as much and as nicely as you could, and for many peasants pea soup (with pork) was a rich, welcome change from the usual turnips and kale. Today, even though Sweden leads the world in atheism, the tradition remains. Pea soup is still a popular dish on Thursdays in Sweden, served in schools, restaurants, and army mess halls.
We don't have pea soup every Thursday ourselves (JV didn't know it was a thing until I found out about it), preferring Pizza Friday and (to a lesser extent) Candy Saturday, but there is usually a tube of ready-made pea soup in the fridge for when we don't feel like cooking. We usually have some bread and cheese to go with it; the soup is so thick that we can just spread it on top of the bread.
We'll be enjoying our semlor today, even though we're not giving up any fatty food for Lent. JV also made some delicious chocolate mousse with tofu and banana. Not quite as traditional but still just as tasty!