On which day did the first child of David and Bathsheba die?

Answer: The seventh day

David fasted and lay on the ground all night pleading with God for the child, but the boy died on the seventh day.

“On the seventh day the child died. But David’s servants were afraid to tell him that the child was dead, for they said, “Look, while the child was alive, we spoke to him, and he would not listen to us. So how can we tell him the child is dead? He may even harm himself.””— 2 Samuel 12:18 (BSB)