day one

noun

1. (often initial capital letters),  the very first day or beginning of something.

Word History and Origins

Origin of day one¹

First recorded in 1975–80

Related Words

• birth 
• creation 
• dawn 
• inauguration 
• inception 
• introduction 
• onset 
• opening 
• outset 
• starting point www.thesaurus.com
• top