Why did Lincoln say, "Fourscore and seven years ago, our fathers brought forth on this continent . . . ?"

He was referring to a very specific date.

He was speaking in the year 1863. Four score and seven (87) years ago was 1776.

So his reference to the nation being "dedicated to the proposition that all men are created equal" was to the Declaration of Independence written four score and seven years earlier.

And then he goes to state that the current conflict is a test of whether a nation founded on that principal can "long endure."

He is talking about slavery without talking about slavery. At least, that is how I interpreted his meaning ever since I first read the speech.