Excuse me Jayway223, but "in (em) the (a) afternoon" is exactly the given translation. So the question still stands: Why is "na tarde" incorrect as compared to "à tarde"? If both are 'correct', can someone explain the difference? Or if it's simply a matter of "that's just how it is", then let me know and I'll just try to fit it into my brain! By the way, I am of the persuasion that Portuguese is just as hard to learn as English, which language I'm thankful I grew up speaking!
à tarde (a+a= à, because it is a feminine expression ) Venha na tarde de domingo ( na = em + a - because tarde here is a determined nom - the next Sunday). Only 'venha na tarde' is not possible. Venha em tardes de domingo. ( 'em' without the article 'a' because 'tardes' are not determined, but you can determine all of them: Venha nas tardes de domingo. Portuguese can be hard, too.