Ès al segón rei romà , Numa Pompilius a qui s'acredita la reforma del calendari. Encara que el calendari va ser reformat dues vegades més després de Numa, primerament per Juli César i molt més tard pel Papa Gregori XIII , Numa va fer els canvis més significatius.

Abans del regnat de Numa, el calendari només tenia 10 mesos. (Alguns diuen que era el mateix Romulus inventar el primer calendari romà!) Això explica perquè els quatre últims mesos de l'any semblen nomenar-se després dels números equivocats: Setè mes, octubre, vuitè mes, novembre, novè mes i desembre, el desè mes. L'any natural tenia 304 dies més una massa imprecisa de 62 dies  l'Hivernus, que curiosament, es considerava un període sense mes.

El 713 aC, el rei Numa, aconsellat pels seus astròlegs, va tenir la brillant idea d'afegir dos mesos a l'any natural, i aquests van ser Januarius i februarius. Tot i que és cert que va ser Numa (si existia, per descomptat) per afegir aquests mesos, hi ha diferències entre els escriptors romans sobre qui va decidir que el mes de gener seria el primer mes de l'any. Algunes fonts afirmen que van ser els Decimvirs a fer-ho en 450 a. C., però altres sostenen que era el propi Numa. Tendeixo a anar amb Numa, simplement pel nom que va triar.

Januarius (January) was named after Janus, god of beginnings and endings. Also god of gates and doorways, he is depicted as having two faces, one looking ahead and one looking behind. This is an apt name for a month that opens and closes the year which is what makes me think it was Numa himself to make this change.

Februarius (February) was named after Februa, a purification festival of Sabine origin that occurred on ides of February (the 15th of the month).

Martius (March) was named for the god Mars. As the god of war, this was Mars’ month because this was the time of year during which, after the cold, wet winter, war would again be taken up. Until Numa’s time, March was the first month of the year.

Aprilis (April) is a bit more complicated as it is not certain exactly from where the name derives. One possibility is that the name comes from the Latin word aperire, to open, as this is the month in which the flowers and plants are opening. To me that seems like a bit of a stretch, so I’m going to go with the other school of thought that would have it that April was named for the goddess Venus (Apru in Etruscan and Aphrodite in Greek), whose festival was on the first of that month.

Maius (May) is more straight forward. It is dedicated to Maia, Greek goddess of the spring and growth. She is the daughter of Atlas, the wife of Vulcan and the mother of Hermes (that is, Mercury, not the most beautiful scarves in the world, although she seems to have one on in this picture).

Junius (June) is clearly named for Juno, one of the three most important gods in the Roman pantheon, and the patron goddess of Rome. She is the goddess of marriage and the well-being of women. She is both the sister and the wife of the chief god, Jupiter. Her Greek counterpart is Hera.

Quintilis (July) originally the fifth month as its name implies, was eventually changed by Augustus in 44 BC to honor Julius Caesar, who had been born during that month. The second major calendar reform took place under Julius Caesar and Augustus carried on this project after Caesar’s assassination.

Sextilis