April 28 marks National Superhero Day, a day when we can celebrate the fictional superheroes we see in movies, television and comic books. But its also a day to recognize the real-life superheroes in our lives and the heroes we dream of becoming.

The holiday has been around for just over two decades, but superheroes have been around for as long as humans have had imaginations. While we think of DC Comics‘ Superman, created by Jerry Siegel and Joe Shuster, as the first real superhero, men and women with extraordinary powers have their roots in mythology, with characters like Robin Hood and Hercules.

Here’s what you need to know about National Superhero Day.

1. The Holiday Was Created by Marvel Comics Employees in 1995

The month of April was a good choice for National Superhero Day, since Action Comics #1 was published on April 18, 1938. However, April 28 is National Superhero Day thanks to Marvel Comics employees.

According to Family Education, Marvel Comics employees started the holiday in 1995 when they had interns ask the public in Pennsylvania about who their favorite superheroes were and what super powers they wanted.

2. There Were Only 200,000 Copies of Action Comics #1 Originally Published

Like the majority of great American success stories, Superman wasn’t expected to be a hit. Jerry Siegel and Joe Shuster created the character in Cleveland and it took them half a decade to find a publisher interested in the character. By 1935, they were already working at National Allied Publications. Initially, they thought Superman would be best as a syndicated newspaper comic strip, but they couldn’t find any syndicates interested. After it was suggested that they take the character to their bosses at National Allied, they found a home for Superman. In March 1938, they sold the character to Detective for $130 and were just happy to know that Superman would be published.

Superman made his debut in Action Comics #1, with several other characters in different stories. As Comichron reports, there were only 200,000 copies published at first. When that run sold out, National Allied surveyed readers and learned that Superman was responsible for the success of Action Comics. By June 1939, Superman had his own solo book and a syndicated comic book strip.

Today, Superman is the star of Action Comics and the self-titled Superman. You can also find him in Trinity with Batman and Wonder Woman, and Justice League. He’s always likely to pop up in DC Comics’ other books.

3. Total Comic Book Sales in North America Passed $1 Billion in 2015

According to data from Comichron, comic book and graphic novel sales reached $1.03 billion in 2015, up from $935 million in 2014 and $870 million in 2013. The site doesn’t have an estimate for 2016 yet.

Of the 2015 total, $90 million came from digital comic book sales, $185 million from comic book store orders of graphic novels and $385 million from comic book store orders of individual comics. Notably, $350 million of that came from book store orders of graphic novels.

“The audience for comics and graphic novels continues to broaden,“ Milton Giepp of ICV2.com said in a statement. “The increase in sales reflects not only the increased awareness of comics properties from other media, we’re also seeing rapid growth in new audiences for comics, including kids and women.”

Notably, the digital comic book sales dropped 10 percent in 2015 compared to 2014.

“Sales of comics in print formats have finally eclipsed the $850 million modern-era high from 1993,” Comichron’s John Jackson Miller said in a statement. “That 1993 total was $1.4 billion in inflation-adjusted dollars, though, so there’s still some ways to go to reach a new inflation-adjusted high for the last 50 years. But the book channel in particular is helping us make progress toward it.”

4. The First Known Use of the Term ‘Superhero’ Dates to 1899

According to Merriuam Webster, the earliest known use of the term “superhero” is 1899.

The dictionary defines the term “superhero” as:

a fictional hero having extraordinary or superhuman powers; also : an exceptionally skillful or successful person

Of course, as Sharon Packer notes in Superheroes and Superegos, characters like superheros have been a part of literature and mythology for centuries. The story of Robin Hood, who stole from the rich and gave to the poor, had the idea of a distinctive costume for the hero. Robin Hood was also a favorite character for early action films, from Douglas Fairbanks’ Robin Hood (1922) to The Adventures of Robin Hood (1938) with Errol Flynn.

The Scarlet Pimpernel of Victorian literature also introduced the idea of a secret identity. Science fiction characters like Edgar Rice Burroughs’ John Carter of Mars also predate comic books. Alex Raymond’s Flash Gordon and Philip Francis Nowlan’s Buck Rogers also came before Superman.

5. Four of the Top 10 Highest-Grossing Movies of 2016 Were Based on Comic Books

Box Office Mojo statistics show that four of the Top 10 highest-grossing films of 2016 were based on comic books.

Disney and Marvel’s Captain America: Civil War grossed $408.08 million in North America alone and it crossed $1.15 billion worldwide. 20th Century Fox’s Deadpool, based on a character from the X-Men films, grossed $363.07 million domestically and $783.1 million worldwide.

Although Warner Bros.’ two DC Comics films from 2016 were both critical disappointments, they were in the Top 10. Batman v. Superman: Dawn of Justice finished its run with $330.36 million domestically and $873.26 million worldwide. Suicide Squad earned $325.10 million domestically and $745.60 million worldwide.

In 2017, there are even more comic book movies on the way. Guardians of the Galaxy, Vol. 2, Wonder Woman, Spider-Man: Homecoming, Justice League and Thor: Ragnarok are all opening this year.