The average man's retirement account contains $139,000. For women, it's $82,000.

Think of all that's happened since the start of the new year — the Oscars, March Madness, failed resolutions. Now imagine if throughout that time, all the men in the US had taken vacation while all the women worked — but the women didn't get paid any more than the men.

That's the concept behind Equal Pay Day, which this year is happening today. Women's equality advocates use Equal Pay Day to symbolize how wide the national gender wage gap is. Last year, the median full-time year-round working US woman...