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).
Quenya
atendëa
double-middle
atendëa
proper name. double-middle
atendëa noun "double-middle", name of the two enderi or middle-days that occurred in leap-years according to the calendar of Imladris (Appendix D, first edition of LotR)