From Tolkien Gateway

Enderi were the 'middle-days' common to the Elven and the Númenórean calendars, and were counted outside the months.

In the Reckoning of Rivendell, three enderi were set between the seasons of Yávië and Quellë. Every twelfth year (except in every 432nd year), these enderi were doubled to six.[1]

In the Kings' Reckoning and Stewards' Reckoning, there was only one enderë; the day of the middle of the year between the astar of Nárië and Cermië was called loëndë. In leap years (divisible by 4 but not 100) it was replaced by two enderi[1]; in years divisible by 1000 loëndë was replaced by three enderi[source?].

In the New Reckoning each year contained three enderi, between Yavannie and Narquelie; the middle of the three was referred to as loëndë.[1]

See also: Lithedays