Equal Pay Day is the date when women's pay symbolically "catches up" with men's earnings from the previous year. This year, April 12 is the big day: the average woman has now made as much money, from Jan 1, 2015 until today, as the average man did by New Year's Eve 2015.
There are a lot of reasons why women earn less than men. PayScale's report, Inside the Gender Pay Gap, show that even in similar roles, controlling for education, experience, and hours worked, women make less money than men. Unconscious bias is a factor, as is "job choice"; women are more likely than men to work at low-paying jobs, and less likely than men to be in leadership roles.