How did international women's day start?

Each year, International Women's Day is celebrated on March 8. The holiday recognizes the importance of women's social and cultural contributions.
According to most historians, International Women's Day-originally called "National Woman's Day"-started on Feb. 28, 1909, in New York. Theresa Malkiel, a suffragette and labor activist, suggested the idea.
Malkiel was Russian-born refugee who fled to America and spent her early years working in a garment factory. She was a member of the Socialist Party of America, and through that organization, she helped to establish the first National Woman's Day to promote equal rights for women (and more specifically, to protest for women's suffrage).
At the time, socialist organizations were some of the only institutions advocating for equal rights for men and women, and many suffragettes were therefore socialist. So to say that International Women's Day was originally a socialist holiday isn't untrue, but it's a bit misleading, since the holiday quickly lost association with the political movement.
In any case, the idea caught on, though the date changed every year. In 1914, Germany recognized International Women's Day on May 8th, possibly because that day was a Sunday. Most countries now observe the holiday on this date.
In 1975, the United Nations began observing International Women's Day, and in 1977, the organization's General Assembly called on member states to proclaim the date as a day "for women's rights and world peace."
Beginning in 2010, the United Nations applied a theme to each International Women's Day. The 2019 theme is "Think Equal, Build Smart, Innovate for Change." As the U.N. explains, that theme "means building smart solutions that go beyond acknowledging the gender gaps to addressing the needs of men and women equally."
Many countries officially recognize International Women's Day, including Afghanistan, Russia, Ukraine, Cuba, Georgia, and Cambodia. However, the United States doesn't officially recognize the holiday; in 1994, Representative Maxine Waters introduced a bill to change that, but Congress never took a vote.