Although annual rituals on May 1 have deep roots in pagan and Christian practices (from maypoles and morris dancing to Easter and celebrations of the Blessed Virgin), this particular reference is to the Communist observance of International Workers’ Day.
The Second International, an association of socialist and labor parties formed in France in 1889, chose May 1 to commemorate the eight U.S. anarchists tried for murder following a bomb-throwing in Haymarket Square, Chicago, during a labor strike and rally in 1886 that killed eight police officers and an unknown number of civilians. (Four of the defendants were hanged, and one committed suicide in jail.)
Parades, especially ones that featured the display of missiles and other large weaponry in Red Square, Moscow, became standard in Communist countries through much of the twentieth century. In Unbearable Lightness, the narrator says state security officers would watch citizens to make sure they attended such parades and regarded them with attentive respect. The photograph here is from a "Victory (in World War II) Day" Parade in Red Square, Moscow -- an event that takes place on May 9, not May Day, but the spirit of the event is similar to the ones described in this novel.
A more detailed description of the May Day parades under the Communists in Prague that Sabina hated appears in section 6 of Part Six, page 249. There, they are labeled “Communist kitsch.”