May 1st is the commemoration known as the International Workers’ Day, or May Day, homage to working people all over the world.
This date was recognized by the majority of the countries, not all of them embracing it as a holiday, but as a festivity. It was voted at the Workers’ Socialist Conference during their second summit, held in Paris, France, in 1889, as a day for vindicating and commemorating the martyrs in Chicago and their struggle for their rights.
They were supposed to be anarchist union workers, who ended up dead in USA due to their participation in strikes for the eight-hour day. The strike started on May 1st, 1886 and reached its Peak 3 days later, on May 4th, with the Haymarket episode, in Chicago.
In the United States, this day is certainly not celebrated as a commemoration. They celebrate Labor Day, every 1st Monday of September, since 1882 with a parade in New York City. This parade is organized by the Knights of Labor.
Back in those years, President Cleveland supported the celebration in September, not in May, has he feared that changing the date would strengthen the Socialist Movement in the USA.