break of day

Dawn, early morning, as in We'll leave at break of day, as soon as it's light, or I feel as though I've been working since the break of day. This term uses break in the sense "burst out of darkness." [First half of 1500s] A synonym from the same period is the noun daybreak.

The noun has one meaning:

Meaning #1: the first light of day
  Synonyms: dawn, dawning, morning, aurora, first light, daybreak, break of the day, dayspring, sunrise, sunup, cockcrow