(often initial capital letters) the very first day or beginning of something.
Origin of day one
First recorded in 1975–80
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for day one
It was a sunny, summer day-one of those days which incline the heart to prayer, and bring tears of happiness to the eyes.Frederick The Great and His Family|L. Muhlbach