13 months

A 13-month calendar is an alternative way of organizing the year into 13 equal periods, each typically lasting four weeks (28 days). This totals 364 days, with an extra day or two added to align with the solar year, creating a more uniform schedule. Unlike the standard 12-month Gregorian calendar, a 13-month system can simplify calculations, planning, and consistency across years. It is sometimes used in certain business, financial, or cultural contexts to promote regularity. However, it requires adjusting traditional date references and is not standard practice worldwide.