Short answers, pulled from the story.
The earliest recorded celebration of International Women's Day occurred on the 28th of February 1909 in New York City. This event was organized by the Socialist Party of America at the suggestion of activist Theresa Malkiel to demand equal rights including the right to vote and the right to hold public office.
German delegates Clara Zetkin and Käte Duncker proposed an annual Women's Day at the 1910 International Socialist Women's Conference in Copenhagen. One hundred delegates from seventeen countries agreed to the proposal though they left the specific date open for national parties to decide.
The date of the 8th of March was cemented into history by the Russian Revolution of 1917 when textile workers in Petrograd began a demonstration that toppled the monarchy. Revolutionary leader Leon Trotsky later wrote that no one imagined this specific day would inaugurate a revolution.
The United Nations began celebrating International Women's Day in 1975 proclaiming it the International Women's Year. The UN General Assembly invited member states to observe the day in 1977 and it has since been commemorated annually with a specific theme.
International Women's Day is an official public holiday in many countries worldwide including Angola, Armenia, Azerbaijan, Belarus, Burkina Faso, Cambodia, China, Cuba, Georgia, Guinea-Bissau, Eritrea, Kazakhstan, Kyrgyzstan, Laos, Madagascar, Moldova, Mongolia, Montenegro, Nepal, Russia, Tajikistan, Turkmenistan, Uganda, Ukraine, Uzbekistan, and Zambia.
In 2024 the theme for International Women's Day was Invest in Women: Accelerate Progress. France became the first nation to guarantee the right to an abortion in their constitution on the 8th of March 2024.