Each year, National Equal Pay Day is determined based on the how far into the year women must work to earn what men worked the previous year. In 2016, that day according to the Department of Labor is today, April 12.
From Pay-Eqauity: Equal Pay Day was originated by the National Committee on Pay Equity (NCPE) in 1996 as a public awareness event to illustrate the gap between men’s and women’s wages.
Since Census statistics showing the latest wage figures will not be available until late August or September, NCPE leadership decided years ago to select a Tuesday in April as Equal Pay Day. (Tuesday was selected to represent how far into the next work week women must work to earn what men earned the previous week.) The date also is selected to avoid avoid religious holidays and other significant events.
Because women earn less, on average, than men, they must work longer for the same amount of pay. The wage gap is even greater for most women of color.