As an FYI, I've never heard nor studied that the National Anthem is a celebration of slavery. This verse is the verse that causes some to question the Star Spangled Banner:
And where is that band who so vauntingly swore
That the havoc of war and the battle's confusion,
A home and a country, should leave us no more?
Their blood has washed out their foul footsteps' pollution.
No refuge could save the hireling and slave
From the terror of flight, or the gloom of the grave:
And the star-spangled banner in triumph doth wave,
O'er the land of the free and the home of the brave.
The reference to, "No refuge could save the hireling and slave." is a reference the British freeing slaves and hiring others in the colonies to join their army as the marched on Washington D.C. to burn it. Thus the emotion of the verse since Washington D.C. was burned. The references today are either taken totally out of context on purpose or people are ignorant of what transpired as Washington D.C. burned.