Translation:Quinta-feira à tarde
á is just the accented syllable, whereas à is a sort of contraction of "á + a" ("a" is the singular feminine definite article).
"à" means "to the."
Correction: à is the contraction of a+a.
The first a is the preposition to and the second a is the feminine singular article the.
I wrote 'quinta de tarde'. It is vaild but I don't really understand why, lol.
dE tarde is the right expression.
dA tarde would mean that the Thursday belongs to the afternoon, which is very weird.
I just made the same mistake. But now that I think of it, I usually hear this phrase when people are making appointments or setting up meetings, so it's more like "Thursday in the afternoon" rather than "Thursday afternoon".
"pela tarde" works. We use it frequently, although in a more formal manner.
what the "a" signify in this context ? Does "Quinta-feira a tarde" mean Thursday in the afternoon ?
Is 'quinta na tarde' correct, or are 'à tarde' and 'de tarde' the only correct translations?
"na tarde" is incorrect. "pela tarde", as @adrianchurches mentioned, is also correct.
Short answer: no.
That sounded a bit rude, so I'll give you the long answer instead: I don't really know why, we just don't say it that way. I don't think "quinta na tarde" is grammatically incorrect, and maybe some people actually use that expression, but it's not usual to say it like that.
I guess it would be a bit similar to ask why is it "in the afternoon" and not "at the afternoon" or "on the afternoon". "At the afternoon" is not completely wrong, but it sounds weird.