13-month calendar

A 13-month calendar divides the year into 13 equal months, each with roughly 28 days, totaling 364 days. This structure simplifies date calculations and makes weeks align consistently within each month. To synchronize with the 365-day solar year, a single additional day (or two in leap years) is added as a special "year" day outside the regular months, or integrated in different ways depending on the system. This calendar aims to create uniform months and improve planning, though it is not widely adopted internationally, and traditional calendars like the Gregorian remain dominant.