Tuesday, May 7th 2012, commonly referred to as Dead Day, is that day that the Atlanta safe zone collapses, indicated by the main power grid going offline, radio broadcasts ordering evacuation, and the inner city's shambler hordes growing to massive sizes. This is considered the beginning of things with the shamblers, i.e. normal life precedes Dead Day whilst the new, apocalyptic one comes after.
The rabies virus had ravaged the midwest all through April, but news reports were censored after the military's airstrike on Minneapolis and St. Paul, Minnesota, in hopes to hold authority in spite of such an atrocity. Eventually, martial rule through the police force is established in Atlanta, separating Allie from her family north of the perimeter. Dahlonega is abandoned and word reaches the city via CB radio, and riots break out at the capital building on April 25th. In reponse, Governor Helley sets up state-sactioned safe zones in the inner city hotels before he flees the city himself.
33 hotels barricade their bottom floors with help from city police and the National Guard before admitting an estimated 1.1 million people; each person is screened individually and those with questionable wounds are turned away whilst any with visible bite marks are summarily executed. The city perimeter is policed by the National Guard and the US Army, but it is overhwlemed at breached on May 7th, at which point a panicked soldier radioes all safe zones with an evacuation order. The resulting pandemonium compromises some of the safe zones' security, even leading to the Hyatt, Hilton, and Sheraton to be demolished. The city's population is mostly killed or turned, which leaves a few survivors left to fend for themselves.