yassë (2) adv. "once upon a time" (YA); writers may rather use yalúmessë or yáressë of similar meaning to avoid confusion with # 1 above.
Quenya
yassë
in which
yassë
once upon a time
yassë
conjunction. where [relative], in which place
yáressë
once upon a time
yáressë noun in locative "once upon a time" (locative form of yárë) (YA)
ya
pronoun. which, that (relative pronoun)
The basic relative pronoun in Quenya, mostly used in inflected forms such as: yassen tintilar i eleni “wherein the stars tremble” (LotR/377). See the entry on relative pronouns for further discusion.
Conceptual Development: The use of ᴱQ. ya as a relative pronoun dates back to the Early Qenya Grammar (EQG) of the 1920s (PE14/54), but for a period in the 1930s and 40s ᴹQ. ya was used more as a remote demonstrative (Ety/YA; PE23/96-98). See the entry on correlatives for more on this usage as of 1948.
ya
which, what
ya (1) relative pronoun "which, what" (attested in VT43:28, 34 and in the Arctic sentence), with locative suffix in Namárië: see #yassë. According to VT47:21, ya is impersonal, "which" rather than "who(m)" (compare the personal form ye). The dative form yan (q.v.) is however used for "to whom" (rather than "to which") in one text, indicating that Tolkien did not always distinguish between personal and impersonal forms. In the phrase lúmessë ya [variant: yá**] firuvammë, "in [the] hour that we shall die", the relative pronoun is not explicitly marked for case and is evidently understood to share the case of the preceding noun (hence not lúmessë yassë**... "in [the] hour in which"...) (VT43:27-28) Presumably, ya has the plural form *yar* (e.g. i nati yar hirnen** "the things that/which I found").
yalúmessë
once upon a time
yalúmessë noun in locative "once upon a time" (locative form of yalúmë) (YA)
#yassë (1) relative pronoun in locative "in which", pl. yassen referring back to a plural noun (relative pronoun ya + locative ending) (Nam, RGEO:66)