hebdomad vs rag week

hebdomad and rag week both are nouns.

Word	Noun	Adjective	Verb	Adverb
hebdomad	Yes	No	No	No
rag week	Yes	No	No	No

As nouns, rag week is a hyponym of hebdomad; that is, rag week is a word with a more specific, narrower meaning than hebdomad:

hebdomad: any period of seven consecutive days
rag week: a week at British universities during which side-shows and processions of floats are organized to raise money for charities

Other hyponyms of hebdomad include week from Monday, rag, Holy Week, Passion Week, shibah, shiva, shivah.

hebdomad (noun)	rag week (noun)
any period of seven consecutive days	a week at British universities during which side-shows and processions of floats are organized to raise money for charities