Plutarch, Quaestiones Romanae
Goodwin, Ed.

Solution. What if it be this (as Juba writes), that on the Kalends the magistrates called (καλεῖν) the people, and proclaimed the Nones for the fifth, while the Ides they esteemed an holy day? Or rather that they who define time by the variations of the moon have observed that the moon comes under three greatest variations monthly; the first is when it is obscured, making a conjunction with the sun; the second is when it gets out of the rays of the sun and makes her first appearance after the sun is down; the third is at her fulness, when it is full moon. They call her disappearance and obscurity the Kalends, for every thing hid and privy they call clam, and celare is to hide. The first appearance they call the Nones, by a most fit notation of names, it being the new moon (novilunium); for they call it new moon as we do. Ides are so called either by reason of the fairness and clear form (εἶδος) of the moon standing forth in her complete splendor, or from the name of Jupiter (Διός). But in this matter we are not to search for the exact number of days, nor to abuse this approximate [p. 217] mode of reckoning; seeing that even at this day, when the science of astronomy has made so great increase, the inequality of the motion and course of the moon surpasseth all experience of mathematicians and cannot be reduced to any certain rule of reason.