In his Epitome of Titus Livy 1, the character of justice and piety, insofar as they exist in the Roman domain, are imagined as connected to worship, divination, and a certain order of time that included the division of days into fasti and nefasti,
Romulus was succeeded (as king of Rome) by Numa Pompilius from the Sabine town of Cures, after (the Romans) had asked him to move to them because of the man’s piety (religio). He taught the rituals (sacra) and ceremonies, and the entire worship of the immortal gods, he instituted pontifices, augurs, the Salii and the other priesthoods; and he divided the year into twelve months, into dies fasti and nefasti; he set up the sacred shields (ancilia) and the Palladium, certain secret sureties of the empire; and Janus Geminus, the sign of peace and war; and the preeminent worship of the hearth of Vesta by her Virgins, so that she would protect the flame of the empire as a likeness of the celestial stars; and he did all this as if at the command of the goddess Egeria.
Now, what are these days which are so all-important? In the eighth book of his lost Prata (cited in Priscian, Institutiones 8), Suetonius gave an etymological explanation:
Dies fasti are those days on which it is lawful to fatur, that is, to speak, as the dies nefasti are those days on which one must not speak.
This does not mean all speech, of course, but only certain specific speech acts. Varro, On the Latin Language 6.53 elaborates:
From (the word fari, ‘to speak’) come the dies fasti, on which it is allowed for praetors to say certain words of legal force without (need for) expiation; from (the same word come) the nefasti, on which it is not lawful to say these and, if they are said, they have to make an expiatory sacrifice.
And further (On the Latin Language 6.29-32):
The dies fasti, on which it is allowed for praetors to use all words without (need for) expiation; […] their opposites are called dies nefasti, on which it is nefas (‘illicit’) for a praetor to say do (‘I give’), dico (‘I declare’) or addico (‘I adjudge’). therefore no action can be taken, because when anything is done by law, it is necessary that (at least) one of these words is used.
Now, if someone has unthinkingly uttered such a word and manumitted a slave, that person is nonetheless free, but with a flaw, as when a magistate who is appointed with a flaw (e.g., unfavorable omens) is nonetheless magistrate. And if the praetor who has said this has done it unthinkingly, it must be expiated with an expiatory sacrifice; if he has spoken the words knowingly, Quintus Maucius said that, as an impious man, he cannot be expiated.
I omit Varro’s discussion of some more complicated variations on the fastus/nefastus distinction because, as important as the dies fasti and nefasti were not just to Roman life but the very conception of time, in the absence of praetors and their legal functions, they are not likely to play a large role in modern practice.
(All translations courtesy of Ɔ. Martiana.)