Roman mythology traced the Republican calendar of 355 days to Numa, the city's second king, and also credited him with the establishment of some kind of intercalation to keep it aligned with the solar year.
There is evidence, however, that the Romans long continued their early and unattested lunar calendar even after the establishment of the Republic.
Alternatively, intercalation is sometimes said to have begun with the decemviri, who may have adopted either Etruscan or Greek practices.
The calendar seems to have been greatly out of sync with the seasons during this period, probably in part due to the exigencies of the Second Punic War.
The details of the law are uncertain, but it seems to have placed the decision whether or not to intercalate a month into the year with the College of Pontiffs.