The term “fatted calf” is used to describe an especially festive celebration, especially one which involves rejoicing about someone's return. This phrase comes from the Parable of the Prodigal Son, a story told in the New Testament of the Bible. In addition to being used to describe a specific type of celebration, a fatted calf can also simply be a particularly zesty and exuberant celebration in which guests have an unusually good time, regardless as to the reason for the celebration.
This term has its roots in the Middle Eastern tradition. Most families who could afford to raise livestock would focus specific attention on a particular animal, fattening it up in anticipation of a special occasion. The slaughter of this animal would be reserved for an especially important event, with other animals being slaughtered to meet the basic food needs of the family. When the fatted calf was slaughtered, people understood that the occasion was particularly notable.
In the Parable of the Prodigal Son, a man has two sons, and the younger son decides to strike out on his own. He travels far, and ultimately ends up leading a broken-down, licentious life which drives him to flee back home. Upon his return, the father slaughters the fattened calf to celebrate, and the older son expresses distaste, wondering why he wasn't rewarded for his loyalty. The father explains that he wishes to celebrate the fact that his son returned to him after being presumed lost, stressing the importance of family bonds.
When someone is planning a particularly ornate party, people may accuse him or her of “putting on the fatted calf” or “killing the fatted calf,” often with a subtle suggestion of reproof which is meant to indicate that the party planning is perhaps getting a bit too complex. People may also specifically say that they intend to put on the fatted calf, indicating that all the stops will be pulled out for the party to ensure that everyone has a good time, and that the event will be memorable. Weddings, for example, may involve a very ostentatious reception party for the guests after the ceremony.
In some cultures, the tradition of fattening up a special animal still endures, and such animals are slaughtered for honored guests and on special occasions. Visitors to cultures where this tradition is retained may want to be aware of this. Having an animal slaughtered in your honor is considered an immense compliment, and people may be offended if you refuse to partake of the meal, or decline special treats which are traditionally offered to the guest of honor at such events.