1.each of the twenty-four-hour periods, reckoned from one midnight to the next, into which a week, month, or year is divided, and corresponding to a rotation of the earth on its axis.
“they only met a few days ago”
synonyms: twenty-four-hour period, full day, twenty-four hours, working day; More
the part of a day when it is light; the time between sunrise and sunset.
“the animals hunt by day”
synonyms: daytime, daylight, daylight hours, hours of light, hours of sunlight, broad daylight, waking hours, the waking day
“you could gamble at night and enjoy the beaches during the day”
antonyms: night
the part of a day spent working.
“he works an eight-hour day”
ASTRONOMY
a single rotation of a planet in relation to its primary.
ASTRONOMY
the period on a planet when its primary star is above the horizon.
archaicliterary
daylight.
“by the time they had all gone it was broad day”
2.a particular period of the past; an era.
“in Shakespeare’s day”
synonyms: period, time, point in time, age, era, epoch, generation
“he was the leading architect of the day”
the present time.
noun: the day
“the political issues of the day”
a particular period in a person’s life or career.
“my student days”
the most active or successful period of a person’s life or career.
noun: one’s day; plural noun: one’s days
“he had been a star in his day”
synonyms: heyday, prime, hour, time, best days, best years, maturity; More
antonyms: decline, nadir
the remaining period of someone’s life.
plural noun: one’s days
“she cared for him for the rest of his days”

Source credit: Google