Titus Livius (Livy) Ab urbe condita 1.19

Having thus obtained the kingdom, he sets about establishing anew, on the principles of laws and morals, the city recently established by violence and arms. When he saw that their minds, as having been rendered ferocious by military life, could not be reconciled to those principles during the continuance of wars, considering that a fierce people should be mollified by the disuse of arms, he erected at the foot of Argiletum a temple of Janus, as an index of peace and war; that when open, it might show the state was engaged in war, and when shut, that all the neighbouring nations were at peace with it.

[2] Twice only since the reign of Numa hath this temple been shut; once when T. Manlius was consul, at the end of the first Punic war; and a second time, which the gods granted our age to see, by the emperor Augustus Caesar, after the battle of Actium, peace being established by sea and land.

[3] This being shut, after he had secured the friendship of the neighboring states around by alliance and treaties, all anxiety regarding dangers from abroad being removed, lest their minds, which the fear of enemies and military discipline had kept in check, should become licentious by tranquillity, he considered, that, first of all, an awe of the gods should be instilled into them, a principle of the greatest efficacy with a multitude ignorant and uncivilized as in those times. But as it could not sink deeply into their minds without some fiction of a miracle, he pretends that he holds nightly interviews with the goddess Egeria; that by her direction he instituted the sacred rites which would be most acceptable to the gods, and appointed proper priests for each of the deities.

[4] And, first of all, he divides the year into twelve months, according to the course of the moon; and because the moon does not make up thirty days in each month, and some days are wanting to the complete year as constituted by the solstitial revolution, he so portioned it out by inserting intercalary months, that every twenty-fourth year, the lengths of all the intermediate years being completed, the days should correspond to the same place of the sun (in the heavens) whence they had set out. ¹ He likewise made a distinction of the days ² into pro- fane and sacred, because on some it was likely to be expedient that no business should be transacted with the people.

[1] Having ¹ 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.