Titus Livius (Livy), The History of Rome, Book 1
Rev. Canon Roberts, Ed.

19. Having¹ in this way obtained the crown, Numa prepared to found as it were anew by laws and customs that City which had so recently been founded by force of arms He saw that this was impossible whilst a state of war lasted, for war brutalised men. [2] Thinking that the ferocity of his subjects might be mitigated by the disuse of arms, he built the temple of Janus at the foot of the Aventine as an index of peace and war, to signify when it was open that the State was under arms, and when it was shut that all the surrounding nations were at peace. [3] Twice since Numa's reign has it been shut, once after the first Punic war in the consulship of T. Manlius, the second time, which heaven has allowed our generation to witness, after the battle of Actium, when peace on land and sea was secured by the emperor Caesar Augustus. [4] After forming treaties of alliance with all his neighbours and closing the temple of Janus, Numa turned his attention to domestic matters. The removal of all danger from without would induce his subjects to luxuriate in idleness, as they would be no longer restrained by the fear of an enemy or by military discipline. To prevent this, he strove to inculcate in their minds the fear of the gods, regarding this as the most powerful influence which could act upon an uncivilised and, in those ages, a barbarous people. [5] But, as this would fail to make a deep impression without some claim to supernatural wisdom, he pretended that he had nocturnal interviews with the nymph Egeria: that it was on her advice that he was instituting the ritual most acceptable to the gods and appointing for each deity his own special priests.

[6] First of all he divided the year into twelve months, corresponding to the moon's revolutions. But as the moon does not complete thirty days in each month, and so there are fewer days in the lunar year than in that measured by the course of the sun, he interpolated intercalary months and so arranged them that every twentieth year the days should coincide with the same position of the sun as when they started, the whole twenty years being thus complete. [7] He also established a distinction between the days on which legal business could be transacted and those on which it could not, because it would sometimes be advisable that there should be no business transacted with the people.