Asscrack of dawn
The asscrack of dawn is the time of day just before the crack of dawn when it is just about still dark; it is the time at which one might get out of bed if attending an urgent appointment at the crack of dawn.
John woke up at the asscrack of dawn with the intent to get to the shops just as they opened for the post-christmas sales in order to avoid getting trampled buy eager bargain-hunters.