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One day before the attack on Pearl Harbor, on Saturday, December 6, 1941, US Naval Intelligence in Pearl Harbor was aware of facts which could have suggested an attack was coming, if someone had recognized their combined implications:
There was a suspicion that the fleet would attack the Malay Peninsula, which in fact it did. This put Japan at war with Britain. (These facts come from the book ``And I Was There'', by Admiral Layton.)
Why attack on Monday? Perhaps just for variety, but there was one possible reason why this attack had to be made on a Monday: if it was timed to accompany another attack in the Eastern Pacific where that same morning would be Sunday.
If there were two synchronized attacks, either of the two could have been scheduled for Sunday. It would be natural to use the extra Sunday factor for whichever attack was more important or needed surprise more. The only attack in the Eastern Pacific that would have been more important than starting a war with Britain would have been an attack on the US. Other possible targets were weakly defended; it would have made more sense to attack them on Saturday so as to save the Sunday factor for the Malay Peninsula.
The target did not have to be Pearl Harbor, but that was one of the most likely places. Of the major US military bases in the Eastern Pacific, Pearl Harbor was closest to Japan and most vulnerable.
None of this was proof, but it could have focused suspicion on the next morning.