Translation:prynhawn dydd Mercher
The main acceptable answers in the database are Prynhawn Mercher and Prynhawn dydd Mercher.
As explained above, it isn't wrong and should be accepted. Try 'prynhawn dydd Mercher' - Duo sometimes objects to capital letters as being incorrect, even though it is not supposed to.
I'm pretty sure it was one of the ones where you pick the correct answers out of three choices. I could've misread it or something, I guess.
The same as in English, I think. With names of days, etc:
- Capital name of the day, but not the words prynhawn, dydd, nos, bore unless at the start of a sentence. (dydd Gwener, nos Fawrth, prynhawn dydd Sul,...)
- Capital name of the month, but not mis, as above. (mis Ebrill, mis Gorffennaf,...)
- Names of seasons are usually lower case. (gwanwyn, hydref,...)
- Names of special days are capitalised, as in English (Noswyl Nadolig, Dydd San Steffan, Dydd Gŵyl Dewi,...)
Simply because people sometimes use the dydd and sometimes they don't. In your own answers you could choose to keep the dydd all the time if it makes it easier for you.
Would "dydd Mercher yn y prynhawn" (no mutation of prynhawn, right?) also be possible?