The name used for leap-days added to the various calendar systems of Middle-earth (LotR/1108, 1112). It is a compound of endë “middle” and the suffixal form -re of ré “day”: ✱enderë “middle-day” pluralized to enderi.
Conceptual Development: In the 1st edition of The Lord of the Rings, a similar term atendëa was used.
enderi noun "middle-days" (sg. *enderë), in the calendar of Imladris three days inserted between the months (or seasons) yávië and quellë (Appendix D)