merrily vs happily

merrily and happily both are adverbs.

Word	Noun	Adjective	Verb	Adverb
merrily	No	No	No	Yes
happily	No	No	No	Yes

As adverbs, merrily and happily are synonyms defined as:

Other synonyms of merrily include blithely, gayly, jubilantly, mirthfully.

merrily (adverb)	happily (adverb)
in a joyous manner	in an unexpectedly lucky way
	in a joyous manner