Having participated in and read the first few threads as the storm approached over the weekend, it was quite clear by Saturday afternoon that this was going to be big, and that it's projected paths were beginning to converge on the New Orleans area. The airlines knew it and cancelled flights. The railroads knew and redirected rail traffic from the area. The President knew and pre-emptively freed up disaster funds for Lousiana and Mississippi (IIRC). The Governor probably knew it as well, but hoped that it would turn away or weaken as many had done before. The mayor knew, but wanted the city to extract a few more dollars from their guests. Or maybe he really was scared of the trial lawyers.
It wasn't until mid-morning on Sunday that the Mayor decided to declare that all should evacuate the city or head for the dome. Strong rumor (and maybe it can be backed up) is that the President told either the Mayor of the Governor that the evacuation had to be ordered.
That was posted on FR. I have this link.