This holiday is most commonly associated as a commemoration of the achievements of the labour movement. The holiday may also be known as May Day or International Worker's Day.
The 1 May date is used because in 1884 the American Federation of Organized Trades and Labor Unions demanded an eight-hour workday, to come in effect as of 1 May 1886. This resulted in the general strike and the Haymarket Riot of 1886, but eventually also in the official sanction of the eight-hour workday.
1 May is celebrated as Labour Day in most countries around the world. In the United Kingdom the bank holiday isn't fixed at May 1st, but instead is observed on the first Monday of May.