I wasn't going to write about this but a (female) colleague suggested I do so.
Equal Pay Day is supposed to be the day "that symbolizes how far into the year women must work to earn what men earned in the previous year." This is hilariously crude statistical analysis. In fact, men and women in the same occupations with similar experience and education actually make almost exactly the same salaries. In some fields, women earn more. The supposed "wage gap" is mostly a function of the choices men and women make in the labor force and has very little to do with discrimination.
Here's a helpful video: