weeknight vs nighttime

weeknight and nighttime both are nouns.

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

As nouns, nighttime is a hypernym of weeknight; that is, nighttime is a word with a broader meaning than weeknight:

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

Other hypernyms of weeknight include dark, night.

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