Short answers, pulled from the story.
The legendary first king Romulus credited with creating the early ten-month Roman calendar. This initial system started in March and ended with December, leaving winter as an unassigned span of days before the next cycle.
Numa Pompilius divided the year into twelve lunar months during his reign as the second king of Rome. He added fifty days to the calendar while removing one day from each thirty-day month to create January and February.
Julius Caesar ordered a complete reformation of the calendar in 46 BC following his victory in civil war. The new Julian calendar measured 365.25 days per year by distributing ten additional days throughout existing months without adjusting nones or ides.
Each month held either 30 or 31 days, creating a total of 304 days for the entire year. These months ran for exactly 38 nundinal cycles which functioned as an eight-day week counted inclusively by Romans.
Control over intercalation became a tool for political gain among Roman magistrates during the late Republic era. Priests manipulated dates to serve their own interests rather than maintain seasonal accuracy, leading to reforms like the 191 BC Acilian Law.