International Socialist Congress, Amsterdam 1904 in the context of Second International (politics)

The Socialist International, commonly known as the Second International, was a political international of socialist and labour parties formed in Paris on 14 July 1889. At a time of growing industrial working-class movements and the expansion of suffrage, it brought together autonomous national parties into a loose international federation. It continued the work of the First International (1864–1876), from which it inherited both the legacy of Karl Marx and the conflict with anarchists. The organization was dominated by the powerful Social Democratic Party of Germany (SPD), whose organizational and theoretical leadership heavily influenced the other member parties.

The International established the annual celebration of International Workers' Day on 1 May and popularised the demand for an eight-hour day. Its early congresses were preoccupied with expelling anarchists and defining its mission as one based on parliamentary political action. After 1900, the International was increasingly confronted with the internal divisions of the socialist movement, particularly the rise of revisionism in Germany and the debate over socialist participation in "bourgeois" governments, sparked by the Millerand affair in France. The 1904 Amsterdam Congress, which saw a major debate between French socialist Jean Jaurès and German leader August Bebel, condemned revisionism and ministerialism, marking the highest point in the influence of the International.

International Workers' Day, also called Labour Day in some countries and often referred to as May Day, is a celebration of labourers and the working classes that is promoted by the international labour movement and occurs every year on 1 May, or the first Monday in May.

Traditionally, 1 May is the date of the European spring festival of May Day. The International Workers Congress held in Paris in 1889 established the Second International for labor, socialist, and Marxist parties. It adopted a resolution for a "great international demonstration" in support of working-class demands for the eight-hour day. The date was chosen by the American Federation of Labor to commemorate a general strike in the United States, which had begun on 1 May 1886 and culminated in the Haymarket affair on 4 May. The demonstration subsequently became a yearly event. The 1904 Sixth Conference of the Second International, called on "all Social Democratic Party organisations and trade unions of all countries to demonstrate energetically on the First of May for the legal establishment of the eight-hour day, for the class demands of the proletariat, and for universal peace".