Happy Birthday, Superman!
I couldn't pass this up, taking notice of the official on-sale date* of the original Action Comics #1, dated June 1938 but on-sale 18 April 1938 -- 75 years ago today.
Cheers! ... Now go read it for a mere 99 cents at Comixology!
* Note: Mike's Amazing World of DC Comics, my customary go-to source for this kind of information, lists the approximate on-sale date as 3 May 1938, but enough other sites are commemorating it today (e.g. here and here) that I'll happily follow suit. And the fact is that the old newstand distribution "system" was so haphazard that I doubt there was any such thing as a meaningful "official on-sale date."