forever vs forever and a day

forever and forever and a day both are adverbs.

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

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

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

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