Etymology of 'nameday'

[ScootPatoot]

I noticed that in many areas, people use the term nameday instead of birthday.

The two seem to be interchangeable, but the only time that birthday is mentioned is in the Azim Steppe, an area which has different beliefs. In this case, the word birthday would refer to the day on which one is born.

Could it be that there were at one time cultural differences that caused the divergence in names, and gradually they grew to mean one and the same? Perhaps at one point, it truly meant the day on which a name was given to a person (in the sight of the Twelve), but later was changed to mean the day on which one was born.

Or might the cultural differences still persist, and now there exist "non-persons" in Eorzea who have no nameday? The term nameday does seem to draw reference from Christianity, after all.

I personally believe the latter, but would like confirmation of if this is the case. But if so, what would be needed for one to be officially named - to have a nameday?