November 22, 1963 (first appeared in 11/22/63)

Long before he wrote 11/22/63, King was referencing the date of JFK's assassination.

November 22, 1963 is mentioned in the following books:

The Dark Tower V: Wolves of the Calla (Nov 2003)

“In any case,” Callahan said, “there’s always been a question as to whether the man who killed him acted alone, or whether he was part of a larger conspiracy. And sometimes I’d wake in the middle of the night and think, ‘Why don’t you go and see? Why don’t you stand in front of that door with the box in your arms and think, “Dallas, November 22nd, 1963”? Because if you do that the door will open and you can go there, just like the man in Mr. Wells’s story of the time machine. And perhaps you could change what happened that day. If there was ever a watershed moment in American life, that was it. Change that, change everything that came after. Vietnam . . . the race riots . . . everything.”
- Part 2 / Chapter IX / Section 19

The Dark Tower VII: The Dark Tower (Sep 2004)

Susannah finds multiple doors through universes, including one to the JFK assassination on 11/22/63.

She opened her eyes and saw doors marked SHANGHAI/FEDIC and BOMBAY/FEDIC and one marked DALLAS (NOVEMBER 1963)/FEDIC.
- Part 1 / Chapter IV / Section 6