Macrobius, Saturnalia 1.7:
“It seems right to me to bring up the origin of the Saturnalia, which pertains not to the arcane nature of divinity, but as it is discussed mixed with fantastic tales or as it is revealed to the common people by natural philosophers. Indeed, the hidden accounts dripping from the font of undistilled truth cannot be narrated even in the sacred rites themselves. If anyone follows them, he must keep them hidden within the confines of his mind. So, our friend Horus may reconsider with me those things which may be known on this topic. Janus once obtained that region which is now called Italy as his kingdom along with Cameses, who equally possessed this land, so that the region is called Camesene, and the town called Janiculum (as Hyginus, following Protarchus Trallanius relates it). Afterward, the kingdom returned to Janus alone. He is thought to have taken on a second face, so that he could see things which are both before and after him. Clearly, this is to be referred to the prudence and wisdom of the king, who knew both past and future things, as Antevorta and Postvorta, obviously the most apt companions in divinity, are worshiped among the Romans. This Janus then received Saturn, carried by ship, as a guest. Janus then learned from Saturn the knowledge of agriculture and improved that rude and savage food before the discovery of grain. He then made him a companion on the throne.”
Saturnaliorum originem illam mihi in medium proferre fas est, non quae ad archanam divinitatis naturam refertur, sed quae aut fabulosis admixta disseritur aut a physicis in vulgus aperitur. Nam occultas et manantes ex meri veri fonte rationes ne in ipsis quidem sacris enarrari permittitur: sed si quis illas adsequitur, continere intra conscientiam tectas iubetur. Unde quae sciri fas est Horus noster licebit mecum recognoscat. Regionem istam, quae nunc vocatur Italia, regno Ianus optinuit, qui, ut Hyginus Protarchum Trallianum secutus tradit, cum Camese aeque indigena terram hanc ita participata potentia possidebant, ut regio Camesene, oppidum Ianiculum vocitaretur. Post ad Ianum solum regnum redactum est, qui creditur geminam faciem praetulisse, ut quae ante quaeque post tergum essent intueretur: quod procul dubio ad prudentiam regis sollertiamque referendum est, qui et praeterita nosset et futura prospiceret, sicut Antevorta et Postvorta, divinitatis scilicet aptissimae comites, apud Romanos coluntur. Hic igitur Ianus, cum Saturnum classe pervectum excepisset hospitio et ab eo edoctus peritiam ruris ferum illum et rudem ante fruges cognitas victum in melius redegisset, regni eum societate muneravit.