It would be off-topic because peeving disguised as a question has been off-topic from day one. And I might add that the way your question is worded right now, it does not even attempt to pretend to know what a disguise is.
As far as I understand from day one means since the very beginning, but, in English, even in day-by-day speech, is it normal to say from day one? And what part of speech is 'day' in that fragment? Adjective?
Best Answer
The word day in day one is a noun.
The expression day one is quite common. This is quite similar to a countdown during a rocket launch, where the last 10 seconds countdown is usually announced "T-minus 10, 9, 8, ..., 3, 2, 1".
Day One usually refers to the first day of some operation or activity. You can even have Day-minus too. For example, the well-known D-Day is June 6, 1944. D-Day Minus One is June 5, 1944. You can say Day One, Day Two, Day Three, and so on.
Also note that you can say T minus 3 minutes, T minus 2 hours, when you want to specified the minus-time not in seconds. Similarly, you can have day minus one month, or in the case of D-Day, D-Day minus two months, and so on.