Whether Job's 'day' was his birthday or not seems to make little difference when you accept the Watchtower explanation for accepting other pagan based practices and obversences. Sending flowers to a funeral, wearing a wedding ring, and using a pinjata at a party are all acceptable to JWs even though they have pagan connections.
Concerning the pinjata, the Awake!, 9-22-03 p23, 24 said, "A main concern is, not what the practice meant hundreds of years ago, but how it is viewed today in your area."
If considering ‘how the practice is viewed in your area,‘ and whether or not the practice has ‘lost the original meaning it had hundreds of years ago’ are acceptable logic with one pagan practice, why can’t the same be applied to other practices that have lost their original meanings, like birthdays?
The number of birthday celebrations where someone is beheaded have gotten to be in the minority; at least where I live.