day one or Day One noun
[singular] : the first day or very beginning of something
▪ We've known this about the project since/from Day One. ▪ The new governor will start implementing these changes on day one. [=the first day of the governor's term]
▪ We've known this about the project since/from Day One. ▪ The new governor will start implementing these changes on day one. [=the first day of the governor's term]
