rag day vs day

rag day and day both are nouns.

Word	Noun	Adjective	Verb	Adverb
rag day	Yes	No	No	No
day	Yes	No	No	No

As nouns, day is a hypernym of rag day; that is, day is a word with a broader meaning than rag day:

rag day (noun)	day (noun)
a day on which university students hold a rag	a period of opportunity
	time for Earth to make a complete rotation on its axis
	a day assigned to a particular purpose or observance
	the time after sunrise and before sunset while it is light outside
	a point or period in time
	the recurring hours when you are not sleeping (especially those when you are working)
	an era of existence or influence
	the time for one complete rotation of the earth relative to a particular star, about 4 minutes shorter than a mean solar day
	the period of time taken by a particular planet (e.g. Mars) to make a complete rotation on its axis