From what I remember on FR Saturday, and perhaps FReepers can find the "evidence" threads (I'm getting some heavy lifting done around the house, you see), Nagin was in close consultation with his lawyers. Apparently, if they ordered a mandatory evacuation, the city would become liable for those who remained during the storm. It took them all that time to figure out that they would have the remaining citizens sign waivers of refusal to remove them of their liabilities, while at the same time calling for an evacuation -- on SUNDAY (a day after it became clear that NO was sitting dead red).
ADD: I looked at Katrina Live Thread III, and Nagin was telling people Saturday afternoon to BEGIN taking precautionary measures, and a VOLUNTARY evacuation went up soon thereafter.
Wow!!
It's in this article:
http://www.nola.com/news/t-p/frontpage/index.ssf?/base/news-4/1125213007249320.xml