August 8, 1961, give or take a few days, Fantastic Four #1 (dated Nov. 1961) hit the newsstands.
While the issue hit the stands on various days on and surrounding that Tuesday most of the surviving copies with any date identification have August 8, 1961 stamped on them. So that is the date that many sources give as the date Fantastic Four #1 was released to the public.
Fantastic Four #1 by Stan Lee and Jack Kirby is generally considered the comic book that started the celebrated Marvel Age of Comics in the 1960s. Marvel Comics, as Timely Publications, began in 1939 but it was the comics created by Stan Lee, Jack Kirby, Steve Ditko, Don Heck, and others in the early Sixties that quickly evolved into the popular “real life” superheroes that formed a FUN pop cultural revolution in the funny book genre.
Of course to start The Age with FF #1 as the first superheroes or comic book of the new era we have to ignore Dr. Droom who appeared five months earlier, or those little “MC” symbols that began appearing on the Martin Goodman/Stan Lee edited floppies four months before the Fab Four debuted.
But it is hard to argue against Fantastic Four being the start of The Marvel Age of Comics.
Like Doctor Droom (who if I recall correctly later appeared in Marvel books renamed as Doctor Druid), the villainous dragon Fin-Fang-Foom had also appeared before FF#1 and was later moved into Marvel continuity, so that’s also a quibble about pushing the date back. (Of course a lot of old Timely characters from the 40s and 50s also eventually wound up in 1961-date Marvel continuity, so it’s a rather feeble quibble.)