In January 2018, news broke that shook the Hollywood industry: after reshoots for All the Money in the World, Mark Wahlberg reportedly earned $5 million for the movie while his co-star Michelle Williams only made $625,000 for her Golden Globe nominated performance.

Mark Wahlberg later donated $1.5 million to Time's Up, a campaign and defense fund to help women cover legal fees when they stand up to sexual harassment and assault, but the money was only one part of the issue. The news also sparked an important conversation about how women are treated in Hollywood, especially after #MeToo.

Sadly, though, that type of pay discrepancy isn't new or unique to Williams' experience with the movie.

"Nationally, the median annual pay for a woman who holds a full-time, year-round job is $41,554 while the median annual pay for a man who holds a full-time, year-round job is $51,640," writes non-profit advocacy group The National Partnership for Women & Families in a new pay analysis.

Researchers have studied why the pay gap exists and they suspect multiple factors, including unequal hiring for gender in the top paying industries (for instance, women only occupy 1/5th of the top-paid STEM jobs, Bloomberg highlighted in 2017) and sexism in salary negotiations.

Meanwhile others speculate that the pay gap is actually a penalty against motherhood. "Harvard economist Claudia Goldin has found that the gender wage gap in America is the largest for women in their 30s — in other words, their prime, childbearing years," reports Vox in a piece which notes "The gender wage gap is really a child care penalty."

But that's where Equal Pay Day comes in.

To try to combat income inequality, Equal Pay Day is a day of advocacy that encourages people all over the world to speak out about the gender pay gap.

But with such a large and persisting issue, you may be wondering what exactly is Equal Pay Day, how does it try to fight income inequality, and how you can participate.

For anyone with questions, here's everything you need to know about it.

What is Equal Pay Day?

Equal Pay day is an annual day of advocacy designed to fight the gender pay gap. The day was started by the National Committee on Pay Equity (NCPE) in 1996, which hoped that, by shining a spotlight on pay discrepancy, communities everywhere could fight to make sure that all people, no matter their gender, get paid equally.

In addition to starting conversations about pay equality, several organizations use the day as an opportunity to call for legislation about the wage gap. For instance, this year The American Association of University Women (AAUW), is calling for:

• The federal government to reinstate the EEOC’s wage data collection, which was halted last year by the U.S. Office of Management and Budget.  Collecting wage data information is critical in order to identify and address gender and racial pay gaps in workplaces.  

• Congress to pass the Paycheck Fairness Act, to update and close loopholes in in the Equal Pay Act of 1963, as well as to pass other pieces of federal legislation that provide additional tools to close the wage gap

• Employers to commit to and implement fair pay best practices, such as conducting regular compensation analyses to ensure equal pay levels and internal equity, setting and publicizing pay ranges for positions, prohibiting the practice of asking job candidates for salary histories, which the AAUW says perpetuates a cycle of lower pay for women, adopting non-retaliation policies for discussing salary, and promoting more women into leadership roles.

The organization also announced a new initiative to train 10 million women in salary negotiations.

When is Equal Pay Day?

Equal Pay Day takes place every year on a Tuesday typically around the first week of April.

The date was chosen for a very specific reason — it highlights the amount of extra work women have to do to earn the same wages as men. "Tuesday was selected to represent how far into the next work week women must work to earn what men earned the previous week," the National Committee on Pay Equity explains on its website.

That's also why Equal Pay Day falls in early April. That's how much additional work women have to do to earn the same yearly salary as men with the current pay gap.

Why does Equal Pay Day exist?

Equal Pay Day was inspired by the persisting wage gap between men and women.

"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," says the National Committee on Pay Equity.

In a new study, The National Partnership for Women & Families highlights exactly what that means for a variety of populations across the United States.

"Nationally, white non-Hispanic women are typically paid 79 cents, Black women 63 cents and Latinas just 54 cents for every dollar paid to white, non-Hispanic men. Asian women are paid 87 cents for every dollar paid to white, non-Hispanic men, although some ethnic subgroups of Asian women fare much worse. The wage gap for mothers is 71 cents for every dollar paid to fathers."

here's what that means in more practical terms. "If the wage gap were closed, on average, a working woman in this country would be able to afford more than one additional year of tuition and fees for a four-year public university, 74 more weeks of food for her family, nearly seven more months of mortgage and utilities payments, or 14 more months of child care."

At the current rate of change, it's estimated that women won't be paid the same as men until 2059, reports AAWU.

Where is Equal Pay Day celebrated?

Equal Pay Day is celebrated, well, everywhere. The day is a grassroots campaign, and as such, relies on individuals, communities, and businesses to plan their own events. Be sure to check out listings in your local community to find something near you.

But Equal Pay Day is not just an American advocacy day, just as the gender pay gap is not just an United States phenomenon. To that end, people all over the globe observe the day -- but not all Equal Pay Days will fall on the same date. Germany, for instance, celebrates Equal Pay Day around the middle of March. "In Germany, it falls on March 18 this year because according to the Federal Office of Statistics, working German men earned 21 percent more than working German women in 2017," explains German broadcast network Deutsche Welle.

Who can get involved?

Anyone — regardless of gender — can participate in Equal Pay Day and stand up to close the pay gap.

How you can honor Equal Pay Day

There are a variety of ways you can participate in Equal Pay Day. First, people are encouraged to wear red for equal pay day. The color was chosen to symbolize how far women and minorities are 'in the red' with their pay, the NCPE explains.

The NCPE also invites people from all over to start a Wage Club in their community. A wage club is a space designed to "allow women to gather regularly to participate in a series of discussions and strategy sessions to help each other take action to close the wage gap in their workplaces."

In addition to that, NCPE suggests several ways to make your voice heard in your local community or your local government including writing an op-ed or letter to the editor and writing to your mayor or local government to officially declare the day as Equal Pay Day. But no matter what you do, the goal is to start conversations about income inequality and work toward closing the pay gap.

And finally, if you are a business owner, the NCPE recommends, in general, that you investigate your company's payment practices throughout the year. "Many employers may not realize their pay scales favor white men as a result of historical and conventional biases and inconsistencies," the NCPE says.