On September 20, in three separate decades, three significant events occurred that changed LGBT history:
1973: Out tennis player Billie Jean King squared off against Bobby Riggs in what the press dubbed the "Battle of the Sexes." King when on to defeat Riggs and made history for women in sports.
1996: President Bill Clinton announced he would be signing the Defense of Marriage Act (DOMA) into law, thus making marriage federally recognized as being only between one man and one woman. At the time Clinton stated, "...this legislation should not, despite the fierce and at times divisive rhetoric surrounding it, be understood to provide an excuse for discrimination, violence or intimidation against any person on the basis of sexual orientation." (Yeah, right!) Clinton later flipped on the issue, and stated he regretted signing DOMA into law.
2011: "Don't Ask, Don't Tell," the ruling that kept lesbian, gay and bisexual people from openly serving in the military, was repealed.
(h/t to Policymic).