It's the first of May!
And, per Wikipedia:
International Workers' Day, also known as Labour Day in some places, is a celebration of laborers and the working classes that is promoted by the international labor movement, anarchists, socialists, and communists and occurs every year on May Day, 1 May, which also coincides with the Celtic spring festival. The date was chosen for International Workers' Day by the Second International to commemorate the Haymarket affair, which occurred in Chicago on 4 May 1886. This Day has its origins in the labour union movement, specifically the eight-hour day movement, which advocated eight hours for work, eight hours for recreation, and eight hours for rest.Or, if your prefer, you can dance around a maypole.
Being a traditional European spring celebration, May Day is a national public holiday in many countries, but in only some of those countries it is celebrated specifically as "Labour Day" or "International Workers' Day". Some countries celebrate a Labour Day on other dates significant to them, such as the United States which celebrates Labor Day on the first Monday of September.