the correct translation, in my opinion, is "à tarde". in order to be a plural, the original sentence should be "in the afternoons". the plural of afternoon is tarde.
For me, it sounds a situation for "em" (em+a=na), but most seem to prefer "a" (a+a=à)? or is it simply because "à tarde" is an adverb meaning "in the afternoon"?
Why is "ele bebe de tarde" wrong here, but "às vezes eu durmo de manha" was correct (as opposed to "da/à manha"? Is it just something that is different with morning, or is there another reason?