May Day!

You can read Wikipedia's history of May Day, here:

International Workers' Day (a name used interchangeably with May Day) is a celebration of the social and economic achievements of the international labour movement. May Day commonly sees organized street demonstrations by hundreds of thousands of working people and their labour unions throughout Europe and most of the rest of the world — though, as noted below, in neither the United States nor Canada. More-radical groups such as communists and anarchists are also given to widespread street protest on this day as well.

May Day was originally the commemoration of the Haymarket protests in Chicago in 1886: in 1889, the first congress of the Second International, meeting in Paris for the centennial of the French Revolution and the Exposition Universelle (1889), following a proposal by Raymond Lavigne, called for international demonstrations on the 1890 anniversary of the Chicago protests. These were so successful that May Day was formally recognized as an annual event at the International's second congress in 1891. The May Day Riots of 1894 and May Day Riots of 1919 occurred subsequently.

In 1904, the International Socialist Conference meeting in Amsterdam called on "all Social-Democratic Party organizations and trade unions of all countries to demonstrate energetically on May First for the legal establishment of the 8-hour day, for the class demands of the proletariat, and for universal peace." As the most effective way of demonstrating was by striking, the congress made it "mandatory upon the proletarian organizations of all countries to stop work on May 1, wherever it is possible without injury to the workers."

May Day has long been a focal point for demonstrations by various socialist, communist, and anarchist groups. In some circles, bonfires are lit in commemoration of the Haymarket martyrs, usually right as the first day of May begins [1].
Due to its status as a celebration of the efforts of workers and the socialist movement, May Day is an important official holiday in Communist countries such as the People's Republic of China, Cuba, and the former Soviet Union. May Day celebrations typically feature elaborate popular and military parades in these countries.

In countries other than the United States and Canada, resident working classes fought hard to make May Day an official holiday[citation needed], efforts which largely succeeded. For this reason, in most of the world today, May Day is marked by massive street rallies led by workers, their trade unions, anarchists and various socialist and communist parties.

Due to its importance in Communist countries, the First and Second Red Scare periods ended May Day as a mass holiday in the United States, which has celebrated Labor Day on the first Monday of September since 1880.