The English used in this article may not be easy for everybody to understand. (March 2019)
The year started on 1 March and had only 304 days or 10 months (March, April, May, June, Quintilis, Sextilis, September, October, November and December). These 304 days were followed by an unnamed 50-day winter period. The Roman king Numa Pompilius (c. 715-673 BC) introduced February and January (in that order) between December and March, increasing the length of the year to 354 or 355 days. In 450 BC, February was moved to its current position between January and March.
In order to make up for the lack of days in a year, an extra month, Intercalaris or Mercedonius, (allegedly with 22 or 23 days) was introduced in some years. In an 8 year period the length of the years were:
1: 12 months or 355 days 2: 13 months or 377 days 3: 12 months or 355 days 4: 13 months or 378 days 5: 12 months or 355 days 6: 13 months or 377 days 7: 12 months or 355 days 8: 13 months or 378 days
A total of 2930 days corresponding to a year of 366¼ days. This year was discovered to be too long, and therefore 7 days were later dropped from the 8th year, yielding 365.375 days per year.
It was the duty of the priesthood not to have any mistake of the intercalary month. However, leap years were considered unlucky. Therefore, they were avoided in time of crisis, such as the Second Punic War. In order to fix the problem, Julius Caesar made his famous calendar reform in 45 BC.