Women's History Month is celebrated in March every year. It traces back to an 1857 protest for women's industrial and work rights, as well as international Women's Day, which began in 1909. In 1981, Congress declared "Women's History Week", which became Women's History Month in 1987. It is a way to encourage gender equality in schools.
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The 2015 theme for Women's History Month is: Writing Women Back into History. See the official Women's History Project website for more information on the theme, or for lots of information on their work!
The official US government site for Women's History month has primary source images and text, audio & visual, and a special teacher's section that links to the Library of Congress, the National Archives, and the National Museum of Art.
History.com has a lot to explore on women's history - from individual biographies to events, to detailed information on the 19th amendment and Title IX.
Time for Kids offers an answer to "why women's history", milestones in women's history, and a look at women in the US military.
The National Register of Historic Places lists many sites relevant to women's history, and often (not always) has ideas on how to bring such places into the classroom. And so does the National Park Service!
The National Women's History Museum is a set of online exhibits available to the public. The museum is in the process of building a site on the National Mall in Washington DC (legislation has passed the House). Online, though, the museum has a series of exhibits with many primary source images and text for educator (and public) use. Just a few examples: Women who Ran for President, Chinese-American Women, Leaving their "stamp" on History, Women in Industry, Women in Jamestown...and many, many more.
The Law Library of Congress has official presidential proclamations on Women's History Month.
It even has its own profile on the US Census site!