I was sending a message to my granddaughter about getting our new car today and put Dydd car newydd ! Is that correct or should it be Diwrnod?
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Dydd is ok here.
Diwrnod is used -
with numbers e.g. tri diwrnod *but see below
with general adjectives, e.g. diwrnod braf, diwrnod gwlyb
when talking about things that fill the day such as work, activities, etc. e.g. diwrnod gwaith, diwrnod gwyliau
Dydd
used with everything else:
days of the week
used with set phrases, e.g: bob dydd, drwy’r dydd, y dyddiau ’ma
*un dydd ar y tro
used for a specific day when not naming the day of the week, e.g Dydd Calan, Dydd priodas
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Diolch 
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