break of day vs first light

break of day and first light both are nouns.

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

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

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

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