Labo(u)r Day is a national holiday in many countries celebrating labor's role in the economy. It's an outgrowth of the industrial era adapted to postindustrialism to serve as a promotion for the political influence of labor unions and a long weekend for public and other institutional employees. It is not an international affair.
International Workers Day, aka May Day, is a global socialist holiday adopted by the Second Internationale in 1889 to commemorate Chicago's Haymarket riot, which occurred several years earlier. Its function is to rally workers to the cause and advance a global socialist agenda hidden behind praise for laboring people.
Did I say Chicago? That's a propos. But never mind.