day

Concise Oxford English Dictionary © 2008 Oxford University Press:
day/deɪ/

▶noun

• 1 a twenty-four-hour period as a unit of time, reckoned from one midnight to the next and corresponding to a rotation of the earth on its axis.

  ■ the time between sunrise and sunset.

  ■ Astronomy a single rotation of a planet in relation to its primary.

• 2 a particular period of the past: the laws were very strict in those days.

  ■ (the day) the present time.

  ■ (days) a particular period in a person's life: my student days.

  ■ (one's day) the successful or influential period of a person's life.

▶adjective working or done during the day.

– phrases
all in a day's work part of someone's normal routine.
any day informal at any time.

■ under any circumstances.

call it a day decide to stop doing something.
day and night all the time.
day by day gradually and steadily.
day in, day out continuously or repeatedly over a long period.
day of reckoning a testing time when the degree of one's success or failure will be revealed. [with allusion to Judgement Day.]
day to day

• 1 happening on a daily basis.

• 2 involving everyday routine.

one day (or some day or one of these days) at some future time.
one of those days a day when things go badly.
that will be the day informal that is very unlikely.
these days at present.

– origin OE dæg, of Gmc origin; rel. to Ger. Tag.