The Royal Navy port-visit reports discussed here describe Pearl Harbor as conspicuously quiet on Sunday mornings. British carriers and other ships visiting during the 1920s and 1930s could observe the routine directly. Those observations provide another route by which a potential attacker could recognize the same vulnerability exposed in American exercises. They support the argument that Japan did not need an exceptional intelligence breakthrough to identify the opportunity. In the British counterfactual, that information again points toward a Sunday attack.
Notable Point · 1:15:07-1:16:01
Pearl Harbor's Sunday routine was public knowledge
Royal Navy port visits had also identified Pearl Harbor's unusually relaxed Sunday mornings, so the vulnerability was observable beyond Japanese or American exercises.