Epag

epag, short for "Epagomenal days," refers to a period of supplementary days added to certain lunisolar calendars. These calendars, which attempt to reconcile lunar cycles with the solar year, often find themselves slightly out of sync. The epagomenal days are inserted to realign the calendar with the seasons and astronomical events. The number of epagomenal days can vary, typically ranging from one to several, and their inclusion is a key characteristic that distinguishes lunisolar calendars from purely lunar or solar systems.

The concept of epagomenal days is not unique to a single culture or calendar system. For instance, the ancient Egyptian calendar incorporated five epagomenal days at the end of the year. Similarly, the Zoroastrian calendar, also known as the Fasli calendar, includes five epagomenal days, called "Gatha days," which are placed at the end of the year. These additional days allow the calendar to maintain its correspondence with the vernal equinox and the solar year, preventing a gradual drift of seasons. The primary purpose of epagomenal days is to maintain the accuracy and practical utility of the calendar, ensuring that festivals and agricultural cycles remain consistent with their intended times.