"How was the weather on Thursday?"
Translation:Sut oedd y tywydd ddydd Iau?
There are two ways to construe the phrase. It could just be "dydd Iau" ("Thursday").
Or it could be considered an abbreviation of "ar ddydd Iau" ("on Thursday"). The preposition "ar" changes a following "d" to "dd".
(I'm confused by why they don't accept "ddydd" in several similar contexts.)
Tywydd ends with the dd sound. From the dd sound, it can be challenging to reposition the tongue into the placement for the single d sound, so dydd can thus be softened into ddydd. I may be wrong here, but this seems consistent with the pronunciation rules I have seen thus far in Cymraeg.