Wiktionary
kill the fatted calf
vb. (context idiomatic English) To begin a festive celebration and rejoicing for someone's long-awaited return.
Usage examples of "kill the fatted calf".
They didn't kill the fatted calf, but there was a young pig didn't see another sunrise.
Let me know when to start the celebration, kill the fatted calf and all that.
The most likely reason you haven't heard from her is that she's shacked up with some Zonee, and sooner or later she'll show up with a few bruises and mental scars and the whole clan can gather and kill the fatted calf.