Originally, to help with the farming cycle, the Romans made four months of 31 days, which they called “full months,” and six months of 30 days which they called “hollow months.” When they added the winter months (January and February) they shorted the hollow months to 29 days and gave January 31 days because, basically, they thought even numbers were unlucky. They gave February 28 days, making it an unlucky month as appropriate for rites of purification and expiation. They dealt with a calendar year of 357 days by squeezing in a month of 27 days after February 24th called Mercedonius which means “work month” every two or three years or whenever it was politically advantageous, and, eventually, a leap day at the end of February, every four years.
Book of Months