Some 37 years after the founding of Rome, which according to tradition was in the year 753 BC, Romulus disappeared in a thunderstorm. The patricians, the Roman nobility, were suspected of having murdered him until Julius Proculus informed the people that he had had a vision of Romulus, who said that he had been taken up to join the gods and was to be worshipped under the name Quirinus.
There was considerable unrest between the original Romans and the Sabines who had joined them after the city was founded over who would be the next king. For the time being it was arranged that the senators should each rule with the king's powers for a period of 12 hours until some more permanent solution could be found. Eventually, they decided that the Romans and Sabines should each elect a king from the other group, i.e., the Romans would elect a Sabine and the Sabines a Roman. The Romans were to choose first, and their choice was the Sabine, Numa Pompilius. The Sabines agreed to accept Numa as the king without bothering to elect anyone else, and a deputation from both Romans and Sabines went off to tell Numa of his election.
Numa didn't even live in Rome, but in a nearby town called Cures. Numa had been born on the very day Rome was founded (21 April), and was the son-in-law of Tatius, a Sabine who had ruled Rome as co-king with Romulus for a period of five years. After Numa's wife died, he had become something of a recluse, and was believed to have been taken by a nymph or nature spirit called Egeria as her lover.
When the delegation from Rome came, Numa refused the position of king at first, but was later talked into accepting by his father and Marcius, a relative, and some of the local people from Cures. They argued that left to themselves the Romans would continue to be just as warlike as they had been under Romulus and it would be better if the Romans had a more peace-loving king who could moderate their bellicosity or, if that proved to be impossible, at least direct it away from Cures and the other Sabine communities.
So, Numa left for Rome, where his election as king was confirmed by the people. Before he finally accepted, however, he insisted on watching the sky for a sign in the flight of birds that his kingship would be acceptable to the gods.
His first act as king was to dismiss the guards Romulus had always kept around. To achieve his aim of making the Romans less bellicose he diverted their attention by means of the religious spectacle of processions and sacrifices and by terrifying them with accounts of strange sights and sounds supposed to come as signs from the gods.
Numa instituted priests (flamines) of Mars, of Jupiter, and of Romulus under his heavenly name of Quirinus. He also added other orders of priests, the pontifices, the salii, and the fetiales, and the vestals.
The pontifices were responsible for public sacrifices and funerals. The salii were responsible for the safety of a shield which had fallen from the sky and was paraded through the city each year accompanied by the salii dancing in armour. The fetiales were peacemakers. Until they agreed that it was a just war, no war could be declared. Originally Numa instituted two vestals, but later increased the number to four. Later still, the number was increased to six by Servius Tullus, the sixth king of Rome. The main duty of the vestals or vestal virgins was to keep the sacred flame alight and to prepare the mixture of grain and salt used in public sacrifices.
Numa also distributed the land conquered by Romulus to poor citizens, hoping that an agricultural way of life would make the Romans more peaceful. He used to inspect the farms himself, promoting those whose farms looked well cared for and as if hard work had been put into them, and admonishing those whose farms showed signs of laziness.
People still thought of themselves first as original Romans or Sabines, rather than citizens of Rome, and to overcome this tendency, Numa organised the people into guilds based on the occupation of the members whatever their origin.
In Romulus' time, the calendar had been fixed at 360 days to the year, but the number of days in a month varied from twenty or less to thirty-five or more. Numa estimated the solar year at 365 days and the lunar year at 354 days. He doubled the difference of eleven days and instituted a leap month of 22 days to come between February and March (which was originally the first month). Numa put January as the first month, and may indeed have added the months of January and February to the calendar.
The month of January is associated with the god Janus, the doors of whose temple were left open in times of war and closed in times of peace. In Numa's reign of 43 years, the doors remained closed, a record.Acknowledgement: Thanks to ylpow for some interesting articles on the Roman calendar before Julius Caesar's reforms.
Plutarch's Life of Numa
Plutarch's comparison of Numa and Lycurgus
Chapters 1.17 to 1.21 of Livy's History of Rome