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 Riot usually right as the first day of May begins.
In the 20th century, May Day received the official endorsement of the Soviet Union; celebrations in communist countries during the Cold War era often consisted of large military parades and shows of common people in support of the government....
more: Wikipedia.org