"we will kill the fatted calf"
The Prodigal Son by Rembrandt
This is a Biblical allusion to the story of the Prodigal Son, as told in the Gospel of Luke (15:11-32). The Prodigal Son asks his father to be given his inheritance early, then goes and spends it all. After he is reduced to absolute poverty, living in a pig pen, the son returns home, expecting his father to be angry. Instead the father throws a lavish party in celebration of his son's return, killing a fatted calf in his honour.
Because of this story, the word 'prodigal' is sometimes misunderstood to mean a repentant person returning home. It actually means 'recklessly extravagant'.
Read the parable of the Prodigal Son here.