The fattened calf was reserved for great celebrations.
"'Take the fattened calf and slaughter it. Then let us celebrate with a feast, because this son of mine was dead, and has come to life again; he was lost, and has been found.' Then the celebration began" (Luke 15:23-24)
For the Jewish people, the fattened calf was choice meat and only reserved for the greatest of celebration days. The father’s choice of meat tells us that the son’s return was worthy of great rejoicing.+
© David Teniers the Younger | Wikipedia PD