Pythagoras
Greek philosopher and mathematician of Samos, founder of a brotherhood that studied number and musical intervals.
c. 570 – c. 490 BCE
Where the months got their names
Macrobius, Saturnalia 1.12–14 (early fifth century CE), in the edition of Franz Eyssenhardt (Leipzig, 1893) · early fifth century CE; the dialogue is set in the 380s
paulo post Numa in honorem imparis numeri, secretum hoc et ante Pythagoram parturiente natura, unum adiecit diem, quem Ianuario dedit, ut tam in anno quam in mensibus singulis praeter unum Februarium impar numerus seruaretur. nam quia duodecim menses, si singuli aut pari aut impari numero putarentur, consummationem parem facerent, unus pari numero institutus uniuersam putationem imparem fecit.
A little later Numa added one more day in honor of the odd number, a secret that nature revealed even before Pythagoras. He gave the day to January, so that the year and every month except February would have an odd number of days. If all twelve months had an even number of days, or all had an odd number, the total would be even. So one month with an even number made the whole count odd.