Essentially it was just timing. Generally the Battleline would spend part of each week at sea conducting drills by squadrons.

However the schedule left all 3 in Pearl that weekend. That also meant their attached escorts of cruisers and destroyers were also at home.

However while large portions were in Port large portions of the Scouting Force were not. 2 carrier task forces were at sea with another in the yards on the West Coast.

As for why with such high tension the fleet was kept in Port, it was with doctrine.

War Plan Orange did not call for any rash immediate counter attacks. It called for the fleet to concentrate at Pearl to begun raids on the Mandated in force after about 10 days or two weeks. Once Japanese focus and location was known.

Thus the fleet was seen as safe in Port. Theoretically no Japanese fleet could surprise it or confront them before ready. Though that was obviously proven false by the shift in strategy brought by Japan.