When an enemy is on red alert awaiting an expected attack, every moment is agonizing, and when the expected attack comes, it's always a surprise.
Nevertheless, the general timing of an attack is always well known if the enemy is paying both attention and spies. The exact moment may not be known, but it doesn't make much difference.
The June 6, 1944 Allied invasion of Normandy was kept pretty much secret due to the very elaborate and well executed ruse the Allies pulled off COMBINED with the Nazi inattention and conviction that their assessment was right. But what was the surprise? An invasion? No. An attack? No. It was just the location and exact timing of the operation.