International Women's Day (IWD) is celebrated on 8 March, commemorating women's fight for equality and liberation along with the women's rights movement. International Women's Day gives focus to issues such as gender equality, reproductive rights, and violence and abuse against women. Spurred by the universal female suffrage movement, International Women's Day originated from labor movements in Europe and North America during the early 20th century.

The earliest version reported was a "Woman's Day" organized by the Socialist Party of America in New York City on 28 February 1909. In solidarity with them, communist activist and politician Clara Zetkin proposed the celebration of "Working Women's Day", approved at the 1910 International Socialist Women's Conference in Copenhagen, albeit with no set date; the following year saw the first demonstrations and commemorations of International Women's Day across Europe. Vladimir Lenin declared 8 March as International Women's Day in 1922 to honour the women's role in the 1917 Russian Revolution; it was subsequently celebrated on that date by the socialist movement and communist countries. The holiday was promoted by the United Nations in 1977.

International Women's Day is a public holiday in several countries. The UN observes the holiday in connection with a particular issue, campaign, or theme in women's rights.

The earliest reported Women's Day event, called "Woman's Day", was held on 28 February 1909, in New York City. It was organized by the Socialist Party of America at the suggestion of activist Theresa Malkiel. There have been claims that the day was commemorating a protest by women garment workers in New York on 8 March 1857, but researchers have described this as a myth.

In August 1910, an International Socialist Women's Conference was organized ahead of the general meeting of the Socialist Second International in Copenhagen, Denmark. Inspired in part by the American socialists, German delegates Clara Zetkin, Käte Duncker, Paula Thiede, and others proposed the establishment of an annual "Women's Day", although no date was specified. The 100 delegates, representing 17 countries, agreed with the idea as a strategy to promote equal rights, including women's suffrage.

The following year, on 19 March 1911, the first International Women's Day was marked by over a million people in Austria-Hungary, Denmark, Germany, and Switzerland. In Austria-Hungary alone, there were 300 demonstrations, with women parading on the Ringstrasse in Vienna, carrying banners honoring the martyrs of the Paris Commune. Across Europe, women demanded the right to vote and to hold public office, and protested against employment sex discrimination.

International Women's Day initially had no set date, though it was generally celebrated in late February or early March. Americans continued to observe "Woman's Day" on the last Sunday in February, while Russia observed International Women's Day for the first time in 1913, on the last Saturday in February (albeit based on the Julian calendar, as in the Gregorian calendar, the date was 8 March). In 1914, International Women's Day was held on 8 March for the first time in Germany, possibly because that date was a Sunday. As elsewhere, Germany's observance was dedicated to women's right to vote, which German women did not win until 1918. Concurrently, there was a march in London in support of women's suffrage, during which Sylvia Pankhurst was arrested in front of Charing Cross station on her way to speak in Trafalgar Square.

On 8 March 1917, in Petrograd (23 February in the Julian calendar), women textile workers began a demonstration that eventually engulfed the whole city, demanding "Bread and Peace"—an end to World War I, to food shortages, and to Tsarism. This marked the beginning of the February Revolution, which alongside the October Revolution, made up the Russian Revolution. Revolutionary leader Leon Trotsky wrote, "23 February (8 March) was International Woman's Day and meetings and actions were foreseen. But we did not imagine that this 'Women's Day' would inaugurate the revolution. Revolutionary actions were foreseen but without a date. But in the morning, despite the orders to the contrary, textile workers left their work in several factories and sent delegates to ask for the support of the strike… which led to mass strike... all went out into the streets." Seven days later, Tsar Nicholas II abdicated, and the provisional Government granted women the right to vote.