A relative conjunction “when” appearing in various phrases in Tolkien’s writings of the 1950s and 60s, a vowel-lengthened form of the relative pronoun ya.
Conceptual Development: Demonstrative, Relative, and Correlative Stems (DRC) from 1948 had ᴹQ. í glossed “(relative) at the time mentioned, at the same time”, a vowel-lengthened form of the relative pronoun ᴹQ. i (PE23/109). This was also given the gloss “when, whenever” in the Quenya Verbal System (QVS) from this same period (PE22/121). Earlier still, ᴹQ. íre was used as the relative conjunction “when” in Fíriel’s Song of the 1930s. In the Early Qenya Grammar it seems ᴱQ. yan “when” served this function (PE14/59).
yá (2) conj. "when" in the sentence yá hrívë tenë, ringa ná "when winter comes, it is cold" (VT49:23). Compare írë #2.