According to this article posted on Bleeding Cool, today is very likely Superman's Birthday!
While the cover clearly states a date of JUNE, 1938, the article goes into greater detail about why April might be a more appropriate month to honor the birth of the Superhero.
@Bleeding Cool said:
What’s the deal? The cover says June 1938, right? Well, yes, but that was an indication for newsdealers as to when unsold copies of the publication were to be removed from the newsstands. The comic arrived at newsstands prior to that. A monthly publication might arrive on the newsstand roughly a month before its cover date. A debut issue might arrive even earlier to attempt to give it a slightly longer sales window.
Superman means a lot of different things to a lot of different people and after 75 years, he's still a polarizing character who represents the best of us in theme and conscience as well as the foundation for a whole genre of entertainment.
Happy Birthday Superman!