A term for the Midyear week appearing in The Etymologies from the 1930s (LotR/LEP, EtyAC/LEP), apparently a combination of enedh “middle” and în “year”.
Noldorin
enedh
noun. core, centre, middle
enedh
noun. centre, middle, core
enedhin
proper name. Midyear week
While the word is written enedh in the Etymologies, it seems that Tolkien considered and reconsidered its form throughout his life. Late rough jottings (c. 1968), as well as the text of letter no. 168 and the fact that the toponym Enedwaith was never changed on the LotR map, seem to imply that ened is the (most) definitive form. See also the possibly related preposition ned , which has sometimes been suggested to mean "in" (while again a prefix nedh- is seen in the Etymologies)