guest night vs eventide

guest night and eventide both are nouns.

Word	Noun	Adjective	Verb	Adverb
guest night	Yes	No	No	No
eventide	Yes	No	No	No

As nouns, eventide is a hypernym of guest night; that is, eventide is a word with a broader meaning than guest night:

guest night: an evening when members of a club or college can bring their friends as guests
eventide: the latter part of the day (the period of decreasing daylight from late afternoon until nightfall)

Other hypernyms of guest night include eve, even, evening.

guest night (noun)	eventide (noun)
an evening when members of a club or college can bring their friends as guests	the latter part of the day (the period of decreasing daylight from late afternoon until nightfall)