silly season vs period of time

silly season and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
silly season	Yes	No	No	No
period of time	Yes	No	No	No

As nouns, period of time is a hypernym of silly season; that is, period of time is a word with a broader meaning than silly season:

silly season: a time usually late summer characterized by exaggerated news stories about frivolous matters for want of real news
period of time: an amount of time

Other hypernyms of silly season include period, time period.

silly season (noun)	period of time (noun)
a time usually late summer characterized by exaggerated news stories about frivolous matters for want of real news	an amount of time