day vs period

day and period both are nouns.

Word	Noun	Adjective	Verb	Adverb
day	Yes	No	No	No
period	Yes	No	No	No

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

Other hypernyms of day include period of time, time period.

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

day: the period of time taken by a particular planet (e.g. Mars) to make a complete rotation on its axis
period: an amount of time

Other hypernyms of day include period of time, time period.

day (noun)	period (noun)
a period of opportunity	a punctuation mark (.) placed at the end of a declarative sentence to indicate a full stop or after abbreviations
time for Earth to make a complete rotation on its axis	the monthly discharge of blood from the uterus of nonpregnant women from puberty to menopause
a day assigned to a particular purpose or observance	an amount of time
the time after sunrise and before sunset while it is light outside	the interval taken to complete one cycle of a regularly repeating phenomenon
a point or period in time	a unit of geological time during which a system of rocks formed
the recurring hours when you are not sleeping (especially those when you are working)	the end or completion of something
an era of existence or influence	(ice hockey) one of three divisions into which play is divided in hockey games
the time for one complete rotation of the earth relative to a particular star, about 4 minutes shorter than a mean solar day
the period of time taken by a particular planet (e.g. Mars) to make a complete rotation on its axis