The Raelian Movement which puts on this annual event is actually a religion that's about cloning, extraterrestrials and sex and freedom.
GoTopless Day falls precisely on Women's Equality Day, Aug 26. It is indeed on Aug 26, 1920 that women earned their right to vote on the basis of Gender Equality. In 1971, the US Congress has made Aug 26 into a nationally recognized date and named it "Women's Equality Day". The president of the United States is summoned to commemorate this date each year.
The reason for the communal baring of breasts in Midtown is 100% legal as New York joined cities around the world in protesting the double standard regarding men's vs. women's nipples, and which of those are ok to display.