At the same time, Aulus Plautius, a very notable senator, went to campaign in Britain, as a certain Bericus pulled out of the island on account of a rebellion and persuaded Claudius to send him some forces. So Plautius took command of his army, but marched with difficulty out of Gaul, as they were displeased about going to war beyond the known world. They did not obey him until Narcissus, who had been sent by Claudius, got up onto Plautius’ platform wanting to give some speech. Then they were so greatly aggrieved by this that they did not let him say anything, and shouted together in a babble that famous saying “Io! Saturnalia!”, since in the Kronia, slaves take on the appearance of masters and celebrate the festival, and right away they became obedient to Plautius.
κατὰ δὲ τὸν αὐτὸν τοῦτον χρόνον Αὖλος Πλαύτιος βουλευτὴς λογιμώτατος ἐς τὴν Βρεττανίαν ἐστράτευσε· Βέρικος γάρ τις ἐκπεσὼν ἐκ τῆς νήσου κατὰ στάσιν ἔπεισε τὸν Κλαύδιον δύναμιν ἐς αὐτὴν πέμψαι. Καὶ οὕτως ὁ Πλαύτιος στρατηγήσας τὸ μὲν στράτευμα χαλεπῶς ἐκ τῆς Γαλατίας ἐξήγαγεν· ὡς γὰρ ἔξω τῆς οἰκουμένης στρατεύσοντες ἠγανάκτουν, καὶ οὐ πρότερόν γε αὐτῷ ἐπείσθησαν πρὶν τὸν Νάρκισσον ὑπὸ τοῦ Κλαυδίου πεμφθέντα ἀναβῆναί τε ἐπὶ τὸ τοῦ Πλαυτίου βῆμα καὶ δημηγορῆσαί τι ἐθελῆσαι τότε γὰρ πολλῷ που μᾶλλον ἐπ’αὐτῷ ἀχθεσθέντες οὔτε τι ἐκείνῳ εἰπεῖν ἐπέτρεψαν, συμβοήσαντες ἐξαίφνης τοῦτο δὴ τὸ θρυλούμενον « ἰὼ σατουρνάλια, » ἐπειδήπερ ἐν τοῖς Κρονίοις οἱ δοῦλοι τὸ τῶν δεσποτῶν σχῆμα μεταλαμβάνοντες ἑορτάζουσι, καὶ τῷ Πλαυτίῳ εὐθὺς ἑκούσιοι συνέσποντο.
—Cassius Dio, Historia romana 60.19.3