After the death of Romulus, the senators could not decide upon whom they would choose to be their next king. Naturally, the Sabines desired a Sabine king, and Romans a Roman one. Since they could not decide, the senators instead devised a complicated plan, by which they would rule Rome in turns. This period of time is called the interregnum, or 'between kings.'
The interregnum only lasted for one year. The people of Rome were not happy, since now they had in essence one hundred kings to rule over them. Wishing to regain the favor of the people before their was a rebellion, the senators decreed that the people could choose their next king, so long as he was ratified by the senate.
The commons proposed Numa Pompilius, a man known for his piety and wisdom. He was also a Sabine. The senate did not want a Sabine king, but there was no candidate of equal virtue to oppose Numa.
True to his nature, Numa's first action as king was to make peace with all of the surrounding towns. He established the Temple of Janus, whose gates were to be open in times of war and closed in times of peace. After the time of Numa, the gates of the Temple of Janus were closed only two times: at the end of the First Punic War and during Pax Romana under Augustus' reign.
With the people at peace, Numa then set about instituting his reforms. He first set up a calendar, dividing the year into twelve lunar months. He next began appointing priests and overseeing the religious duties they were to perform. The highest of these was the Pontifex Maximus, or 'chief priest.' He also brought to Rome the cult of the Vestal Virgins, which was already practiced in Alba Longa (see the story of the founding of Rome).
With these and other religious reforms, Numa diverted the attention of the people of Rome away from war and towards more civilizing influences. He was in virtually every way the exact opposite of his predecessor Romulus, yet was equally successful. In the 43rd year of his rule, Numa Pompilius died of natural causes.
Livy. Ab Urbe Condita.