Women in the United States were given the right to vote on August 26, 1920 when the 19th Amendment to the United States Consitution was certified.  The amendment was first introduced in 1878.  Every president in the United States has made a proclamation for Women's Equality day since 1971 when legislation was introduced in Congres by Bella Abzug.

Bella Savitsky Abzug was an American lawyer, Congresswoman, social activist and a leader of the Women's Movement. In 1971she joined Gloria Steinem and Betty Friedan to found the National Women's Political Caucus. She declared "This woman’s place is in the House—the House of Representatives." She became the first Jewish woman in the United States Congress. She was later appointed to chair the National Commission on the Observance of International Women's Year.

Joint Resolution of Congress, 1971
Designating August 26 of each year as Women’s Equality Day

WHEREAS, the women of the United States have been treated as second-class citizens and have not been entitled the full rights and privileges, public or private, legal or institutional, which are available to male citizens of the United States; and

WHEREAS, the women of the United States have united to assure that these rights and privileges are available to all citizens equally regardless of sex; and

WHEREAS, the women of the United States have designated August 26, the anniversary date of the passage of the Nineteenth Amendment, as symbol of the continued fight for equal rights: and

WHEREAS, the women of United States are to be commended and supported in their organizations and activities,

NOW, THEREFORE, BE IT RESOLVED, the Senate and House of Representatives of the United States of America in Congress assembled, that August 26th of each year is designated as Women’s Equality Day, and the President is authorized and requested to issue a proclamation annually in commemoration of that day in 1920, on which the women of America were first given the right to vote, and that day in 1970, on which a nationwide demonstration for women’s rights took place.

This is a copy of a real stamp - just think about it. Women weren't allowed to vote.