Women's Day is celebrated to honor the achievements of women throughout history. As to the reason why it is celebrated on March 8th specifically, we will have to go back nearly 100 years in history.
In 1910, the International Socialist Women's Conference suggested March 8 as the day to commemorate Women's Day. Later Soviet Russia adopted it as a national holiday in 1917 to recognize the sacrifices made by women during World War I. In 1975, it was further ingratiated by the UN as the official International Women's Day.