The Autumn Equinox: it's a holiday of many names.
None of them—to be perfectly honest—quite there yet, if you know what I mean.
Equinox, of course, comes from Latin: “equal night.” It has the advantage of being readily comprehensible, at least. The down side is, of course, that it's ambiguous, since it's got a twin in the spring. And somehow it's got that clinical sound to it.
Then there's Evenday. This is a modern loan-translation from the word for “equinox” in the Scandinavian languages. (Interesting that, to describe a time when day and night are of equal length, the Southrons focus on night and the Northrons on day; make of that what you will.)
“Evenday” has a nice, colloquial sound to it, and is probably relatively transparent to anyone with light behind the eyes. Interestingly, it has already developed two pronunciations, and (curiously) I find myself using both of them: Even-day and Even-dee, just like the days of the week: the formal and less formal options, respectively.
Wishing folks a “Happy Evenday” has a good sound to it, certainly. But, of course, there's still that vernal-autumnal ambiguity.
So far as we can tell, the ancient Kelts did not observe the sunsteads and evendays as holidays (focusing instead on what we would call the “Cross-Quarters”), so there were no traditional names for them in any of the Keltic languages. To rectify this situation, Druidic Revivalists in the 19th century coined Welsh names for them; the autumn evenday is now called Alban Elfed (supposedly, “Light of [the] Waters”), and the name has gained a certain currency in Druidic circles.