Short answers, pulled from the story.
International Women's Day originated on the 28th of February 1909, when the Socialist Party of America held a "Woman's Day" event in New York City, organized at the suggestion of activist Theresa Malkiel. The following year, German delegates Clara Zetkin, Kate Duncker, and Paula Thiede proposed making it an annual international event at the 1910 International Socialist Women's Conference in Copenhagen.
The date was fixed at the Second International Conference of Communist Women in 1921, when a representative named Nikolaeva proposed the 8th of March in memory of the Petrograd women's demonstrations of that date in 1917, which helped trigger the Russian Revolution. A Bulgarian delegation had proposed a single universal date, and the proposal for the 8th of March passed unanimously.
On the 8th of March 1917, women textile workers in Petrograd walked out and launched demonstrations demanding "Bread and Peace," which historian Leon Trotsky later identified as the start of the February Revolution. Within seven days of the protests, Tsar Nicholas II abdicated and the provisional government granted women the right to vote.
The United Nations began celebrating International Women's Day in 1975, the year it had proclaimed as International Women's Year. In 1977, the UN General Assembly formally invited member states to observe a day for women's rights and international peace, with the 8th of March becoming the standard date.
International Women's Day is an official public holiday in countries including Russia, China, Ukraine, Cuba, Kazakhstan, Mongolia, Nepal, and several others. In Germany, only Berlin and Mecklenburg-Western Pomerania have made it a public holiday, while some countries such as Australia, France, and Italy observe the day widely without it being an official holiday.
The Feminist Five were five Chinese women, Li Maizi, Wu Rongrong, Zheng Churan, Wei Tingting, and Wang Man, who were detained for thirty-seven days in 2015 after planning to distribute anti-sexual harassment stickers ahead of the 8th of March. Their detention drew international protests across multiple countries, and they were released on bail on the 13th of April 2015.