Why was the USA so unprepared for the attack on Pearl Harbor in 1941?

Extract from a report by a British naval intelligence officer

Catalogue ref: WO 208/2081

In case the Japanese should endeavour to use the same technique in an attack on Singapore or any other British possession where they are present in numbers too great for internment, these notes of their methods in the attack on Pearl Harbour may be of use. I cannot vouch for the accuracy of the stories, some are from eye-witnesses and they are in any case given rather as examples of the means which might be used in any future attack than as historically accurate narrative.

1). Date chosen for attack.
Saturday night and Sunday morning are Honolulu’s most vulnerable times, as most social events are on Saturday nights and most officers can be expected to be away from their commands.

Advertisements appeared in the Honolulu newspapers for some time before the attack and the authorities discovered (after the attack) that by placing a mask (like a stencil) over these advertisements, the words which showed through the holes cut in this mask gave the date and hour of the attack in addition to other information.

Certain, quantities of ammunition were brought out of store a week or two before the attack. A few days before it took place, orders were received “from high quarters” to replace this ammunition. These orders were carried out.

For some day before, Japanese girls had been making "dates" with sailors for that Saturday night, and most of them saw that the sailors were well filled up with liquor. This was remarkable because it is apparently unusual for a Japanese girl to mix with sailors. Also a Japanese restaurant keeper near Pearl Harbour gave drinks on the house. On Friday many Japanese quit their jobs and did not turn up on Saturday.

On that particular Saturday the men of the Fleet received their Christmas bonus.