I believe that what you are referring to is the speech that Douglass gave on the Fourth of July in 1852 in Rochester, New York.
In this speech, he is contrasting the meaning of the day for blacks and whites. Basically, he says that white people should celebrate the day. To them, it should mean independence and joy.
He says that, for whites, it is a day of celebrating the
rich inheritance of justice, liberty, prosperity, and independence
that white people got from the founding fathers.
Douglass says that whites should greet the day with joy because it is a day of life and healing for them, even if it is not a day that has brought any joy to blacks.