Tuesday is Equal Pay Day—so dubbed by the National Committee for Pay Equity, which represents feminist groups including the National Organization for Women, Feminist Majority, the National Council of Women's Organizations and others. The day falls on April 12 because, according to feminist logic, women have to work that far into a calendar year before they earn what men already earned the year before.

In years past, feminist leaders marked the occasion by rallying outside the U.S. Capitol to decry the pernicious wage gap and...