forever and a day vs forever

forever and a day and forever both are adverbs.

Word	Noun	Adjective	Verb	Adverb
forever and a day	No	No	No	Yes
forever	No	No	No	Yes

As adverbs, forever and a day and forever are synonyms defined as:

forever and a day and forever: for a very long or seemingly endless time

forever and a day (adverb)	forever (adverb)
for a very long or seemingly endless time	for a limitless time
	for a very long or seemingly endless time
	invariably