The celebration of Fat Thursday is on the last Thursday before Ash Wednesday and the beginning of the Christian season of Lent.
Lent is a time of fasting, which means abstaining from foods that contain animals with red blood (meats, poultry, game) and products from animals with red blood (milk, cheese, eggs, etc.), and fish and seafood with backbones. The number of meals on each day is also limited.
In days gone by when the Church had more influence over the people and they lived by the rules, Lent put a certain pressure on them because they had to rid their households of all those forbidden things that might spoil.
So they ate and partied up until the last Tuesday before Ash Wednesday, or Ostatki .
One of the tradtional foods made to use fats and eggs was Paczki.
Today Fat Thursday is more of a celebration than preparation for Lent. It is actually an excuse to gorge on Polish Donuts - Paczki.
Note that the name Mardi Gras is French for Fat Tuesday. That is celebrated as is Ostatki in Poland. A final party before Lent.
On Fat Thursday Polish people descend on their favorite bakeries to buy copious amounts of Pazki. They take them to the offices and put out plates piled with more than anyone would normally consider eating. Especially since they normally have a 400 calorie load.
The average consumption is 2 1/2 per person in Poland. In some offices and at parties, there are often contests at which the winner often eats 10 or more of these calorie bombs.