ok I'll give a reply a try. Although there are much better minds and more informed people this is the argument I have used.
I personally believe that Job's children were celebrating their birthdays. However, there is now foolproof way to prove that to a JW. The WT has a QFR on this and basically discredits the whole idea based on the fact that the word Hulledeth (sp?) doesn't precede the word yowm (sp?). Basically saying that the word "day" without the word "birth" in front of it cannot mean birthday. Looking at the context shuts that arguement down pretty well for me and apparently several other translators and bible commentators.
First, if the celebration of birthdays had been wrong or looked down upon; Job's children wouldn't have even been celebrating them. They were righteous like their father.
Second, if celebrating birthdays was wrong in some way then Job wouldn't have sacrificed in their behalf "in case one of them sinned in their heart". He would have sacrificed because they had in fact sinned by celebrating their birthday or doing wrong things at their party.
Third, the brothers were the ones who feasted on their days and the fact that they invited their daughters shows that it "most likely" (wt phrase) wasn't a drunken gluttonous crazy butt party. I forget which commentator brought this out but in fact since the sisters were invited it showed how nice everyone got along and how it was probably a nice family occcasion.