first light vs break of day

first light and break of day both are nouns.

Word	Noun	Adjective	Verb	Adverb
first light	Yes	No	No	No
break of day	Yes	No	No	No

As nouns, first light and break of day are synonyms defined as:

Other synonyms of first light include aurora, break of the day, cockcrow, dawn, dawning, daybreak, dayspring, morning, sunrise, sunup.

first light (noun)	break of day (noun)
the first light of day	the first light of day