What's my number riddle

Question

The first and the last of the days of the week,
I and Ζ they are numbers in Latin and Greek.
In N and in P, and period and group,
Of them I consist and I always have been
But now in my name, what number is seen?

Answer 1

Partial answer - all the clues seem to point to the number:

Seventeen

The first and the last of the days of the week

...are numbered 1 and 7, respectively

I and Z in Latin and Greek:

I is 1 in Roman numerals, and Z is 7 in modern Greek

In N and in P, and period and group:

Looking at the periodic table of elements, Nitrogen (N) is in period 2, while Phosphorous (P) is in group 15. 2 + 15 = 17

Of them I consist and I always have been:

If this statement specifically refers to the first half of this sentence, "I" could be taken to mean Iodine, which is in group 17 of the periodic table.

But now in my name, what number is seen?

Your username is "David", so I think we're looking for numbers in there.

But that's as far as I can figure out at the moment, since:

"D" could refer to the Roman numeral (500) or the Greek (4), "V" could refer to Roman numeral (5) or Vanadium (group 5, period 4), "I" could refer to the Roman numeral (1) or Iodine (17) etc.