Great Britain vs Boxing Day

Great Britain and Boxing Day both are nouns.

Word	Noun	Adjective	Verb	Adverb
Great Britain	Yes	No	No	No
Boxing Day	Yes	No	No	No

Great Britain (noun)	Boxing Day (noun)
an island comprising England and Scotland and Wales	first weekday after Christmas
a monarchy in northwestern Europe occupying most of the British Isles; divided into England and Scotland and Wales and Northern Ireland; `Great Britain' is often used loosely to refer to the United Kingdom