field day vs period of time

field day and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
field day	Yes	No	No	No
period of time	Yes	No	No	No

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

Other hypernyms of field day include period, time period.

field day (noun)	period of time (noun)
a day for outdoor athletic competition	an amount of time
a day devoted to an outdoor social gathering
(military) a day for military exercises and display
a time of unusual pleasure and success