Quote Originally Posted by MistakeNot View Post
There is nothing to indicate that nameday and birthday mean the same thing, is there?
Nameday seems to reference the day a person got their name, which may or may not be the same day they were born.
Whereas the term "birthday" is only used in a singular quest in all of FFXIV (Welcome to the Family), the term "nameday" shows up at least five or six dozen times and as far as I can tell, is used in a way that we, IRL, would use "birthday".

Galfrid: "You may as well be strolling around in your nameday suit. Come back when you've acquired some sturdier gear."
Alphinaud: "Well, if you recall, you yourself emerged from the Lifestream as naked as your nameday. It is a scene I shall never forget─"

There are also nameday gifts, choosing your nameday in CC, and a variety of other instances where the phrase pops up. While this could have been a different day from your actual birth date at one point in history, the way in which the term is used today appears to be synonymous with one's actual birthday. If anything, this might be simply hashed up to the Localization team using Nameday as a fantasy replacement term for birthday, similar to sennight for week despite an Eorzean week being 8 days.