Other forms: days
A day is the twenty-four hours from one midnight to the next, and also the light hours between night and night. All day, the next day, a lovely day.
The prepositions are fixed: on Monday, in the day, during the day, all day. These days means nowadays; one day means both a past occasion and a future one, and the tense tells you which. Note the day before yesterday and the day after tomorrow, which English says in four words where many languages have one. And day in, day out means continuously and wearily.
1noundayA period of twenty-four hours.~88%
“It took three days and two of them were wasted.”
2noundaylightThe hours of daylight.~6%
“The days are getting shorter already.”
3nounparticularA particular period in the past.~6%
“In my grandfather's day nobody locked anything.”
Shares are an estimate: a language model read 17 sentences found in the wild and placed 17 of them on a sense — a count of our sample, not a measurement of English.
day is a period of time.
Old English daeg goes back to a root meaning to burn — a day is the hot part. It is unrelated to Latin dies, which gives diary and journal, so English and Latin named the same thing from two different observations.
Middle English
day
English, today
day
From Middle English day. Middle English day, from Old English dæġ, from Proto-West Germanic dag, from Proto-Germanic dagaz (“day”), from Proto-Indo-European dʰegʷʰ- (“to burn”). 2
Etymology from Wiktionary, CC BY-SA