1st May known as Labour 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.

May Day Procession in Queensland May 1953 Source: Wikimedia