May 1, the International Workers' Holiday or Labor Day is an official Holiday in Bulgaria.

May 1 commemorates the fight for the eight hour workday. The idea for a "workers holiday" began in Australia in 1856.

In 1889, with a resolution of the Second International, May 1 became an official celebration to commemorate the victims of the so-called Haymarket Affair, and was marked for the first time in 1890.

The Haymarket affair occurred during the course of a three-day general strike in Chicago, Illinois that involved common laborers, artisans, merchants, and immigrants. Following an incident in which police opened fire and killed four strikers, a rally was called for the following day at Haymarket Square. The event remained peaceful, yet towards the end of the rally, as police moved in to disperse the event, an unknown assailant threw a bomb into the crowd of police. The bomb and resulting police riot left at least a dozen people dead, including seven policemen. A sensational show trial ensued in which eight defendants were openly tried for their political beliefs, and not necessarily for any involvement in the bombing. The trial lead to the eventual public hanging of seven anarchists. The Haymarket incident was a source of outrage from people around the globe.

As such, May Day has become an international celebration of the social and economic achievements of the labor movement.

The first marches in Bulgaria were organized in 1893 in Sofia, Plovdiv and Vidin. May 1 has been declared official holiday in 1939 and is celebrated each year since 1945.

Although May Day received its inspiration from the United States, the US Congress designated May 1 as Loyalty Day in 1958 due to the day's appropriation by the Soviet Union. Alternatively, Labor Day traditionally occurs on the first Monday in September in the United States.

May 1, the International Workers' Holiday is celebrated all over the world and is an official holiday in Germany, Austria, Mexico, Thailand and Russia among others.