One of the dog days of summer: a late-summer day.

One person's working time for a day, or the equivalent, used as a measure of how much work or labor is required or consumed to perform some task.

A period that was a relatively long time ago; the distant past.

For a very long or seemingly endless time.

(computing, idiomatic) (of vulnerability) newly discovered, and therefore still not fixed and possibly exploited by hackers or other criminals

(computing, idiomatic) vulnerability that has been discovered recently, and is yet unpatched or unmitigated; zero-day vulnerability

plural of day trip

The day on which a President and a Vice-President of the United States takes office, now on January 20 after a Presidential election, before 1936 on March 4.

(US politics) The day on which general elections are held in the United States: the Tuesday following the first Monday in November.

(US, slang) Thanksgiving Day; Turkey Day.