*** Rbi8 Job 1:4 ***
And his sons went and held a banquet at the house of each one on his own day; and they sent and invited their three sisters to eat and drink with them.
To give more credence to the thought that Jobs’ sons were celebrating their birthday we can look at Job 3:1-3
*** Rbi8 Job 3:1-3 ***
3 It was after this that Job opened his mouth and began to call down evil upon his day. 2 Job now answered and said:
3 “Let the day perish on which I came to be born,
Also the night that someone said, ‘An able-bodied man has been conceived!’
His sons held a banquet ‘each one on his own day’. Job ‘began to call down evil upon his day’. What day? ‘Let the day perish on which I was born’.
It is hard to argue against the fact that Job allowed his children to celebrate their birthdays.