An epagomenal day is a day in a calendar which stands outside the usual groupings of weeks or months.
Epagomenal days appear in solar calendars as a way to fill the gap between a system of months or weeks of equal lengths and the actual length of a solar year. In the ancient Egyptian calendar, 12 months of 30 days were followed by 5 such days. In the Mayan calendar, 18 months of 20 days are followed by five epagomenal days designated Uayeb to round out the year to 365 days. In the Bahá’í calendar, 19 months of 19 days, have four or five days added to make the year 365 or 366 days long.