May 1 - an important holiday in socialist and communist countries - actually commemorates the Haymarket riots in Chicago, May 4, 1886. What began as a strike by workers demanding an eight-hour workday quickly led to a bombing in Haymarket Square and a battle between workers and policemen. Several people died; protesters were jailed. Three years later the Second International - a loose federation of socialist groups and labor unions that influenced the European labor movement - proclaimed May Day as International Workers' Day.
Today the holiday celebrates worker solidarity; in some countries workers use the occasion to protest.
In Moscow, hundreds of thousands of workers march through Red Square to proclaim solidarity of workers worldwide.