Middle-days

The Elves divided the year into six periods or seasons. Four of these were fifty-four days long, while summer and winter (Lairë and Hrívë) were each longer, at seventy-two days. These six seasons, together with the additional days of Yestarë and Mettarë at the beginning and end of the year, gave a total of 362 days, three short of a true solar year. To address this disparity, a further three days were added to the year, the Enderi or 'middle-days'. These lay outside any of the seasons proper, between Yávië and Quellë (that is, in mid-autumn, precisely halfway through the Elves' year). On a modern calendar, the middle-days would normally have fallen across the three days from 25 to 27 October each year.

The middle-days were also used to address the need for leap years. Rather than adding a single day every four years as with a modern calendar, every twelfth year the Elves doubled the Enderi to a total of six days, accomplishing the same end of synchronising their calendar with the actual movement of the Sun. This approach was not exactly precise, and it is said that further refinements were also used (for example, every third yén - that is, every 432 solar years - the extra three middle-days that would normally be added were omitted, so keeping the calendar closely accurate).

The concept of middle-days or Enderi was also used by the Númenóreans and their descendants, though the details of their mechanism was rather different. Rather than using a twelve-year cycle like the Elves, the Dúnedain preferred a four-year cycle like a modern calendar. Rather than simply inserting an extra day, they instead replaced their Midsummer day, known as Loëndë, with a pair of middle-days and thus extended the leap year's length to 366 days.