Why is the capitalised version incorrect? Bore Dydd Gwener should be just as OK as Bore dydd Gwener as indeed bore dydd gwener should be as well. Unless we are discussing names the capialisation is somewhat irrelevent.
Yes agreed, Duolingo seems to be inconsistent in its treatment of case, sometimes ignoring it and sometimes insisting on it.
I've added all possible variations to avoid confusion with this sentence.
Thanks for pointing it out.
Why has Duo started omitting dydd from the answers? Is it not needed all the time?
Have a look at the tips and notes for the "Days" lesson; this is explained there.