When it begins. (1) With sun-set: The Jews in their “sacred year,” and the Church—hence the eve of feast-days; the ancient Britons “non dierum numerum, ut nos, sed noctium computant, ” says Tacitus—hence “se'n-night” and “fortnight;” the Athenians, Chinese, Mahometans, etc., Italians, Austrians, and Bohemians.
(2) With sun-rise: The Babylonians, Syrians, Persians, and modern Greeks. (3) With noon: The ancient Egyptians and modern astronomers. (4) With midnight: The English, French, Dutch, Germans, Spanish, Portuguese, Americans, etc.
“Sewing as she did, day in, day out.” —W. R. Wilkins: The Honest Soul.
“Old Joe, sir ... was a bit of a favourite ... once; but he has had his day.” —Dickens.
I have lost a day (Perdidi diem) was the exclamation of Titus, the Roman emperor, when on one occasion he could call to mind nothing done during the past day for the benefit of his subjects.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894