"It was very warm on Sunday."
Translation:Roedd hi'n dwym iawn ddydd Sul.
Historically it would have been "ar ddydd Sul" i.e word for word on "on Sunday", but in modern speech the "ar" is often dropped so you have two options, show that there was once a reason for a mutation to occur by including the mutation i.e "Roedd hi'n dwym iawn ddydd Sul", or since there is now no reason for a mutation to occur you can chose not to mutate it. Most people either mutate and include the preposition or mutate it by itself.
"Roedd hi'n braf iawn ar ddydd sul" = Most of the time, "braf" is commonly used as a north Wales dialect, but can be understood in all of Wales as well.
braf is is usual Welsh word for 'fine' weather - it is not a dialect word.
Strictly, ar ddydd Sul means 'on Sundays'. '...on Sunday' is just ...ddydd Sul.
However, ar is also used with a particular date - ar ddydd Sul y 18fed (on Sunday the 18th)