Hi, Rootofallevil, as you can see from the response of AlmostAtheist your question is not an easy one to answer, precisely because the word as we use it today "birthday" - does not occur at Job 1:4. The reason is because there is no such word in the Heb. The way the Heb describes this concept is with the expression 'Yom ha-Yalad' which literally means "day of the birth" This phrase occurs at Gen 40:20, where that brilliant Heb "scholar" Freddie Franz "translated" that phrase as Pharaoh's "birthday" lit "Yom ha-Yalad" - day of his birth.
Now let us try and answer your question - using the WTS favourite buzz word "reason" Let's now "reason" on this matter. What we want to know is: what does the expression at Job 1:4 mean when it says "his own day" - "yom" - Right?
Well Job 3:1 uses the expression "His day" - Like Job 1:4 it is the word "Yom" - day - What "day" was Job referring to when he spoke of "his day" ???? hmmmm?
Let's look at the very next verses, at Job 3:3, actually. Here Job tells us that he was speakng of his "Yom ha-Yalad" !!!!!! ie "the day of his birth" or [yup you guessed it] his "birthday" !!!!!!
So - "day" of Job 3:1= "Yom ha-Yalad" ie "day of [one's] birth"- "birthday"
Therefore, I submit that "day" of Job 1:4 = "day of [one's] birth" - "birthday"
Actually there is at least one translation to my knowledge which uses "birthday" at Job 1:4 and it is the New Living Translation
Incidently, if Freddie could "translate" the expression "Yom ha-Yalad" as "Birthday" at Gen 40:20, then why did'nt he do the same at Job 3:3, [or for that matter why not at Ez 16:4 as well]
So as you can see, though we cannot be absolutely dogmatic about this matter [indeed we certainly ought not to be] we can be "reasonably" certain that textually the Heb WAS referring - in some way - to those kid's birthdays Of course the WTS will always protest [methinks they protest too loudly probably knowing how weak their case is] by quoting Prof So-and-So But believe me, I would rather be us than them on this matter. We've got several of the finest scholars to back us up [William Gesenius, Jameson Fausett and Brown, Matthew Henry, etc] plus something devastating - the freedom to think for ourselves
Hope this helps
Cheers