I was thinking of a sentence;
July 16, 1999, is the date at which my dog died.
It shows the specific date at which my dog died.
But when I use "a" instead of "the", I think I am focusing more on the type of the date.
July 16, 1999, is a date at which my dog died.
As is the case with the day:
That was a day my dog died (type, or kind of the day)
That was the day my dog died (identifying the day; the specific day)
Am I correct about the articles involving the date?