Today is International Worker's Day. It's not widely celebrated in the US despite the fact that it commemorates a Chicago strike

May 1st is an official holiday in over 80 countries. In countries that it's celebrated, workers usually get the day off, and gatherings are held to commemorate the advances of the worldwide labor movement. A curious thing about this day, though, is that it's NOT widely celebrated in the United States. The irony is that the holiday is celebrated May 1st because of a strike that occurred in Chicago in 1886. Here's what happened:

In May of 1886, there was a labor demonstration at Haymarket Square in Chicago. The rally was peaceful strike advocating for an eight-hour workday. However, at some point, an unknown person threw a dynamite bomb at the police. This startled the police and the gunfire that followed resulted in the deaths of 7 police officers and at least four civilians. Many more were injured. 

After these event, the Labor movement organized demonstrations on the anniversary of the Chicago protests. May Day was recognize as an annual event at the second congress of the Second International, a labor organization with ties to socialist movements. This resulted in the 1894 May Day riots. In 1904, the International Socialist Conference called on all their supporters to demonstrate on May 1st in favor of an 8-hour workday. These efforts eventually led to many countries recognizing May 1st as an official Holiday.

In the United States, however, because of the day's ties to socialism and the international labor movement, May 1st has been a controversial subject. Different efforts to try to move Labor Day to May 1st have failed.  Despite this, many organizations like Latino Immigrant groups and Occupy Wall Street choose May 1st as a day of protest. 

(Source)