According to my welsh boyfriend, no one says " Bore Mercher". It just doesn't make sense. It is incomplete. So there is a difference. You have to say "Bore dydd Mercher", you can't exclude the "dydd"! I'm not sure if things are different in the south. This is coming from the North of Wales.
It must be regional. Like i said, just where i live in North Wales, they always use the "dydd". Not saying it applies to the whole country, just where i live. Apparently, it is unusual to exclude it in this area. Just odd. The only time i have heard "dydd" excluded is when referring to the night, which makes sense i suppose.
I have to ask :) At least 3 (sometimes more) of the questions in the general 'strengthen skills' are about 'Dydd Mercher' in one form or another, almost every time. There are tons of other words and phrases that could pop up but never do. Why this obsession with Dydd Mercher? :)