Hiroshima, August 6th, 1945.
From "Pica-Don" animated short film by Renzo Kinoshita (1978).

First known peer-reviewed work in history is Descartes Meditations, 1641. Before publishing he sent it to many scholars for comment. He wrote a long appendix detailing the "referees" objection, and presumably changed some of his wording and claims based on the "referee reports".

100 years of Quantum Mechanics: George Uhlenbeck and Samuel Goudsmit postulated in 1925 the idea of intrinsic angular moment of the electron, which we now call spin. Three years later it would be incorporated into Dirac’s relativistically covariant theory of fermions (the “Dirac equation”).