Still doesn't explain why "Day" and not "Phase" or "Stage" or "Level" or "Period"

I think I covered the basic explanation in my article as well as dzone and gitlab did in theirs.

:D

I think "day one" was an existing phrase that was confusingly extended to cover other "start of phase" concepts.

Sounds like a nice way to trick your management though, "Uh no, it won't have feature X on day one, but that will be working by Day-2."

I'll admit I've never heard of day 0/1/2 before.