Quenya 

atendëa

proper name. double-middle

The name of the leap-days used to adjust the Númenórean calendar in the 1st edition of The Lord of the Rings (RC/728). It is a combination of at(a)- “double”, endë “middle” and the adjective suffix -a. In the 2nd edition, the name was changed to the enderi (LotR/1108).