Why does the father decide to kill a fatted calf in the story the parable of the prodigal son?

The father in this Parable missed his younger son very much and when the son came home the father was over joyed and he called for a robe and a ring and then the fatted calf. The fatted calf was reserved for special times, like a sacrifice or a great feast celebration and the return of his son call for great celebration.

The overall theme of this story is the joy of God, and the celebrations that occur in heaven when a sinner comes to God and repents and is given salvation.