Translation:I saw the weather forecast on Friday.
Since this sentence is referring to a particular Friday, is ar an Aoine an equivalent substitute for Dé hAoine? I had thought that ar an Aoine would have been preferred for a generic Friday, i.e. “on a Friday” rather than “on Friday”, despite the an.
EDIT: It is an equivalent substitute. The NEID gives an example of ar an Aoine referring to a particular Friday in the past:
the airlift of troops took place on Friday tharla aeraistriú na dtrúpaí ar an Aoine
The first noun does not need to be defined with an article "an" because it is already defined by the defined genitive "na haimsire".
Hata cait = a cat's hat (both undefined) An hata cait = the cat hat, as in the kind of hat a cat would wear (only the hat is defined) Hata an chait = the hat of the cat (both defined, so no need for a defining article at the start ...) Hata don chat = a hat of the cat (only the cat is defined)