The Sumerians, Babylonians, and ancient Egyptians had calendars based on the lunar month. The Egyptians, however, adjusted their system so that in each year the same months would fall in the same seasons. They fixed the year at 12 months of 30 days each, with five extra days a year belonging to no month. This gave them a year of 365 days, one quarter of a day short of a full solar year. To make up for this quarter day, they established a leap-year system, providing an extra day every four years.
The Roman calendar under the republic had 12 months, with 355 days in a year. Each new year was begun on the date when the Romans' consul, who was elected annually, took office. Until 153 B.C., consuls took office on March 1; thereafter, on January 1.
The first day of the Roman month was called the Calends, or Kalends. From this word we get calendar. The Ides was the 15th day in months of 31 days and the 13th day in months of fewer days. The Nones was the 9th day before the Ides. The Romans reckoned time backwards, as so many days before the Ides, Nones, or Calends.
The Roman high priests (called pontifexes, or pontiffs) had charge of the official calendar. It was their task to add an extra, or intercalary, month about every two years to make the Roman calendar come out even with the solar year of 365 1/4 days. But the pontiffs abused this power. For political reasons, such as making an official's term shorter or longer, they sometimes would neglect to add the intercalary month or would add too many of them.