Women's History Month started as only one week in 1978; the Education Task Force of the Sonoma County (California) Commission on the Status of Women planned a weeklong celebration of women's history coinciding with International Women's Day, which is March 8. The movement gained popularity across the country and several women's groups advocated for national recognition. President Jimmy Carter designated the week of March 8 to be National Women's Week. In 1987, Congress passed Public Law 100-9, which recognized the month of March as Women's History Month.
Information paraphrased from the National Women's History Museum.