What ancient festival did masters temporarily serve slaves?

Many festivals utilized the "backwards day" device. The one often cited is the Yule or Christmas festivals in medieval England and Scotland. It followed in the footsteps of the Roman's Saturnallia. A "Lord of Misrule"was appointed from the servants who acted like he was the lord or master, the master waited on the servants and all that.

Just in case you are wondering, the masters did not suffer from amnesia the next day. Servants who got carried away by abusing the master or his family either physically or by mocking them to crudely would find themselves in the master's bad books for a long time.