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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

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First Known Use of DAY ONE

1971

Related to DAY ONE

Synonyms: alpha, baseline, birth, commencement, dawn, beginning, genesis, get-go (also git-go), inception, incipience, incipiency, kickoff, launch, morning, nascence, nascency, onset, outset, start, threshold

Antonyms: close, conclusion, end, ending, omega

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