It should be mentioned that the most powerful figures in Gotham were present to hear the Joker's announcement, including the mayor, Commissioner Gordon, and District Attorney Lando Calrissian ...
What the hell are they looking at, anyway?
You'd think that, after being provided with such generous advance notice, Gotham PD would put themselves on high alert and cover the entire downtown area. We don't know how big the area is, but how hard could it be to spot the guy in the clown makeup traveling on a parade float, blaring Prince music and dragging a couple of gigantic balloons behind him?
And yet, the first we ever see of the cops is like 10 minutes later, after the Joker has had enough time to poison several people, shoot down Batman's plane, and take his girlfriend into a bell tower. They knew the parade would be at midnight. Why were they so late?
"Goddamn daylight saving time!"