The correlatives ᴹQ. tanar and tanaryas appeared in Demonstrative, Relative, and Correlative Stems (DRC) from 1948 (PE23/109), a combination of ᴹQ. ta “that” and ᴹQ. -ar(yas) “day”.

Conceptual Development: Fíriel’s Song from the 1930s had untranslated ᴹQ. táre in the phrases ᴹQ. ananta úva táre fárea, ufárea “but yet it will not be enough, not enough” and man táre antáva nin Ilúvatar, Ilúvatar? “what will the Father, O Father, give me” (LR/72). It likely means “*[on] that day” = ᴹQ. ta + ᴹQ. are; compare ᴹQ. enyáre “in that day” = ᴹQ. enya + ᴹQ. are from the same poem.

Neo-Quenya: For purposes of Neo-Quenya I would update this to ᴺQ. tanaurë “on that day” using Q. aurë for “day”.