Treiglad adferfol and the lack thereof
There appears to be a general lack of treigladau adferfol, i.e where there first adverbial phrase is mutated: see part 6.46 of 'Gramadeg y Gymraeg', Peter Wyn Thomas (t. 431-433).
'I went to the pub Friday ' SHOULD be 'Es i i'r dafarn ddydd Gwener' not 'dydd Gwener'.
Correct :) I personally tried to revise as many sentences as possible where the treiglad adferfol is used after the original sentences were written by another contributor but some might have been missed. Keep flagging them up and the team will pick them up and correct them :)
It has said in the notes for a long time that both are used by Welsh speakers and are therefore accepted, although the Welsh course does swap between forms more than most courses on Duolingo.