Well, I'm not too sure about the exact position of the ACLU, but...
I am fine with displays on private property, but when it is on government property it's a different story.
So, we have a nativity scene up on Springfield Town Hall. Now the Satanists want to make a decoration right next to that. The gov't agrees, but the next night, they find the Satanist decoration vandalized. The atheists now want to put up an Ode to Darwin. Christians complain that the atheist's decorations have more words on their decoration than their own. Atheists counter point is that the Christian decoration has more letters on it. The town hall's parking has been completely taken over by decorations, and is no longer able to conduct business. The satanists now allege the christians destroyed their decoration, and state that the Christian's decoration is slightly larger in area than their own. The christians say that the satanist decoration is slightly taller than their own. The scientologists now want to have their decoration flying over all of the other decorations. The christians oppose saying it will cut off the sunlight. The atheists agree to it, only if the scientologists agree to lessen the weight of their decoration by 436 pounds to match that of the atheist's decoration.
Either that or...
Religious decoration is banned on government property and allowed on private property.