What is Labour Day or May Day, and why is it celebrated on May 1?

The decision to mark May 1 as May Day was taken in memory of Chicago’s Haymarket Affair, which took place in 1886, during a protest in support of workers' rights. The incident resulted in the death of seven police officers and at least four civilians.
May 1 is marked as May Day, also referred to as International Workers’ Day and Labour Day in different countries across the world. The Day commemorates the contributions of workers and the labour movement.

Although observed as an ancient northern hemisphere spring festival initially, May 1 became associated with the labour movement in the late 19th century, after trade unions and socialist groups decided to designate it as a day in support of the working class people.