Dictionary

day one

noun, often capitalized D&O

: the first day or very beginning of something

Full Definition of DAY ONE

:  the first day or very beginning of something

Examples of DAY ONE

  1. We've known this about the project since Day One.
  2. <since day one of this project, we've always insisted that we would create the best product possible without any consideration of cost>

First Known Use of DAY ONE

1971

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