Significant dates in labour history.
- May 01 - In 1884 Proclamation of the demand for eight-hour workday in the United States. Two years later, in 1886, the general strike which eventually won the eight-hour workday in the United States, began. These events are today commemorated as May Day or Labor Day in most industrialized countries; Thomas Lewis died; the 1946 Pilbara strike occurred in Australia; International Woodworkers of America merged with the International Association of Machinists; Mike Watson was born; the Taksim Square massacre occurred in Turkey in 1977
- May 02 - Nazi Germany outlawed free trade unions and established the German Labour Front; Bernice Fisher died
- May 03 - The International Typographical Union was founded; the Bay View Tragedy occurred in 1886 in the U.S.
- May 04 - Haymarket Riot occurred in 1886 in the U.S.
- May 05 - John Sweeney was born; Jackie Presser stepped down as Teamsters president due to cancer; James Duncan was born
- May 06 - Miguel Contreras died; Frank Fitzsimmons died; Ludvik Buland was born
- May 07 - Miguel Contreras was born; Maurice Hutcheson was born; David Sullivan was born
- May 08 - Jerome Wurf was born; the Hard Hat riot occurred in the U.S. in 1970
- May 09 - Elias Motsoaledi died; 1934 West Coast Waterfront Strike began in the U.S.
- May 10 - Isaac Theophilus Akunna Wallace-Johnson died; Walter Philip Reuther died; the 2008 Skorpion Zinc Strike began in Namibia
- May 11 - William Konyha was born; Pullman Strike began in 1894 in the U.S.
- May 12 - Coal Strike of 1902 began in the U.S.; the American Maritime Officers was founded
- May 13 - Henk Sneevliet was born
- May 14 - Arthur Moore (labor leader) was born; the Ådalen shootings occurred in Sweden in 1931
- May 15 - Pope Leo XIII issued Rerum Novarum; Winnipeg General Strike of 1919 began; George Mock stepped down as president of the Teamsters after eight days; the Ulster Workers' Council Strike began in 1974 in Northern Ireland; the U.S. Supreme Court decided Gompers v. Buck's Stove and Range Co.; Arthur Creech Jones was born
- May 16 - The Minneapolis General Strike of 1934 began in the U.S.; the Iraqi Federation of Trade Unions was founded; the U.S. Supreme Court decided NLRB v. Mackay Radio & Telegraph Co.; A. Philip Randolph died
- May 17 - The first Starbucks Workers Union was organized; former trade union leader Francisco Largo Caballero was deposed as prime minister of Spain
- May 18 - Bill Haywood died; the Atlanta transit strike of 1950 began in the U.S.
- May 19 - James P. Hoffa was born; the Battle of Matewan began in 1920 in the U.S.
- May 21 - Cyrus S. Ching was born
- May 22 - Agustín Tosco was born; the Steel Workers Organizing Committee was disbanded in 1942; the United Steel Workers of America was founded in 1942
- May 23 - The "Battle of Toledo" occurred during the Auto-Lite Strike in 1934 in the U.S.
- May 24 - The Pennsylvania Association of Staff Nurses and Allied Professionals was founded; Peter J. Brennan was born
- May 25 - Philip Murray was born; the Remington Rand strike of 1936–1937 began in the U.S.; Basdeo Panday was born; Will H. Daly was born
- May 26 - Actors' Equity Association was founded; the Ohio Federation of Teachers was founded
- May 27 - The U.S. Supreme Court decided In re Debs
- May 29 - The Disney animators' strike began in 1941 in the U.S.; the Cordobazo uprising began in 1969 in Argentina; the 2006 TTC wildcat strike began in Canada
- May 30 - William Sidell was born; the Memorial Day massacre of 1937 occurred in the U.S.; the U.S. Supreme Court decided Lehnert v. Ferris Faculty Association
- May 31 - W. A. Boyle died