(often initial capital letters) the very first day or beginning of something.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
How to use day one in a sentence
And so, love, Tuesday is to be our day—one day more—and then!The Letters of Robert Browning and Elizabeth Barrett Barrett, Vol. 1 (of 2) 1845-1846 | Robert Browning and Elizabeth Barrett Barrett
Till one day—one day—the tiger that was tamed falls upon the tamer, the master, the lover, the friend!The White Plumes of Navarre | Samuel Rutherford Crockett
And that day one year my Jock smacks me on the cheek, and gets on the mare; and when I ask: 'Where be goin'?'The Trespasser, Complete | Gilbert Parker
The clock struck; I suddenly remembered that it was on this hour and this day one year ago that my mistress deceived me.Child of a Century, Complete | Alfred de Musset
No; I have not forgotten what this day one year ago took from you, or what it gave to me—my heart's best treasure.Elsie's New Relations | Martha Finley