nighttime vs weeknight

nighttime and weeknight both are nouns.

Word	Noun	Adjective	Verb	Adverb
nighttime	Yes	No	No	No
weeknight	Yes	No	No	No

As nouns, weeknight is a hyponym of nighttime; that is, weeknight is a word with a more specific, narrower meaning than nighttime:

nighttime: the time after sunset and before sunrise while it is dark outside
weeknight: any night of the week except Saturday or Sunday

Other hyponyms of nighttime include wedding night.

nighttime (noun)	weeknight (noun)
the time after sunset and before sunrise while it is dark outside	any night of the week except Saturday or Sunday