First impressions redux

Question

Once upon a time,
a child wrote a rhyme.
It asked for the way,
you can date this day,
if we know the moon,
and its springtime tune.

Answer 1

@Amit Naidu has already given a good explanation for why the hidden answer is what it is; please give their answer your upvotes too.

As for the steganography aspect:

The answer is hidden in the lines of the poem.
It can be found by looking at the first word of each line, counting the number of letters in it,
and then taking the $n^{th}$ letter of the $n^{th}$ word of each line:

Once upon a time, $\hspace{8.5ex}$ E    Once = 4 letters; 4th letter of 4th word ("time") = E
a child wrote a rhyme. $\hspace{4ex}$ A    a = 1 letter; 1st letter of 1st word ("a") = A
It asked for the way, $\hspace{5ex}$ S    It = 2 letters; 2nd letter of 2nd word ("asked") = S
you can date this day, $\hspace{4ex}$ T    you = 3 letters; 3rd letter of 3rd word ("date") = T
if we know the moon, $\hspace{5.7ex}$ E    if = 2 letters; 2nd letter of 2nd word ("we") = E
and its springtime tune. $\hspace{2ex}$ R    and = 3 letters; 3rd letter of 3rd word ("springtime") = R

And so we have our answer confirmed at last.

Answer 2

You must be talking about:

Easter, because it is Easter weekend and its date in the Gregorian calendar is based on the spring equinox and the ecclesiastical full moon.

I couldn't find the referenced child, but here is the rhyme by English poet Justin Richardson (1900-1975):

No need for confusion if we but recall That Easter on the first Sunday after the full moon following the vernal equinox doth fall.

There is a similar Scottish poem too, but it does not employ the vernal equinox as its reference.