Since 2004, May 2nd is the official Polish Flag Day. The Polish flag consists of two horizontal stripes: white on the top and red on the bottom. Variation of the flag displaying a crowned, white eagle national coat of arms on the white stripe is flown abroad and at sea. Read more about the Polish flag here.
White and red were officially adopted as national colors in 1831. They are of heraldic origin and derive from the tinctures (colors) of the coats of arms of the two constituent nations of the Polish-Lithuanian Commonwealth, i.e. the White Eagle of Poland and the Pursuer of the Grand Duchy of Lithuania, a white knight riding a white horse, both on a red shield. Prior to that, Polish soldiers wore cockades of various color combinations. The national flag was officially adopted in 1919. Since 2004, Polish Flag Day is celebrated on May 2.
The flag is flown continuously on the buildings of the highest national authorities, such as the parliament and the presidential palace. Other institutions and many Polish people fly the national flag on national holidays and other special occasions of national significance. Current Polish law does not restrict the use of the national flag without the coat of arms as long as the flag is not disrespected.
May 2nd is a second public holiday after the May 1st, currently known as State Holiday, before the May 3rd, which is a Polish Constitution Day.
Besides the important religious holidays like Christmas, Easter, and Corpus Christi, the three day long May holiday is the longest public holiday in Poland. Even the reminiscence of the old, communist International Labor Day of May 1st, won’t shadow the obvious benefits of the couple days off from work.
So if you sit in traffic in Chicago and notice Polish flags on cars and trucks, now you know why.