Paczki Day is a Polish tradition in which people use up lard, sugar and eggs in preparation for Lent, which begins the day after.
Paczki (pronounced POONCH-key), or pączek, is described by Dictionary.com as follows: "A round Polish pastry similar to a doughnut, usually filled with fruit and topped with sugar or icing."
Paczki Day is particularly popular in American cities with large Polish populations such as Chicago, Buffalo, and Detroit.
The Polish American Journal provides further information including recipes (small, large, old Warsaw, or old Polish) to prepare paczki yourself.
Celebrations are being held across the Midwest to honor the day and local bakeries are making extra paczki to accommodate Paczki Day.