Plutarch Caesar 59

[1] The adjustment of the calendar, however, and the correction of the irregularity in the computation of time, were not only studied scientifically by him, but also brought to completion, and proved to be of the highest utility. For not only in very ancient times was the relation of the lunar to the solar year in great confusion among the Romans, so that the sacrificial feasts and festivals, diverging gradually, at last fell in opposite seasons of the year,

[2] but also at this time people generally had no way of computing the actual solar year; ¹ the priests alone knew the proper time, and would suddenly and to everybody’s surprise insert the intercalary month called Mercedonius. Numa the king is said to have been the first to intercalate this month, thus devising a slight and short-lived remedy for the error in regard to the sidereal and solar cycles, as I have said in his Life. ²

[3] But Caesar laid the problem before the best philosophers and mathematicians, and out of the methods of correction which were already at hand compounded one of his own which was more accurate than any. This the Romans use down to the present time, and are thought to be less in error than other peoples as regards the inequality between the lunar and solar years. However, even this furnished occasion for blame to those who envied Caesar and disliked his power. At any rate, Cicero the orator, we are told, when some one remarked that Lyra would rise on the morrow, said: Yes, by decree, implying that men were compelled to accept even this dispensation.