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.
Redonkulously early in the morning. Often it is still dark out. Commonly used when one must wake up early after a night of drinking, or getting up to go on a trip. Also can be referred to "Oh dark hundred"
Person 1: "Dude, what time do we have to get up tomorrow?"
Person 2: "The freaking ass crack of dawn."