(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
Related Words for day onegenesis, birth, outset, dawn, kickoff, opening, introduction, inauguration, creation, onset, inception, top, ingress, dawning, start, initiation, outbreak, foundation, entrance, origination
Examples from the Web for day one
Historical Examples of 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