Wiktionary
crack of dawn
n. (context idiomatic English) The first moment of daylight; sunrise.
Usage examples of "crack of dawn".
If Caro was at a costume ball, she would drink too much and have too sore a head to leave tomorrow morning at the crack of dawn, as they must if they were to make it back to Hampshire by nightfall, as promised.
Her husband was a civil engineer of like outlook, out since the crack of dawn supervising the laying of a new water main.
After a littlethought, he directed the mare to the castle where he could command a bed for a few hours and set out for Stirling at the crack of dawn next day.
I'd planned to surprise everybody by showing up at the Tate place at the crack of dawn, ready to travel.