There's actually been a new "canon" timeline posted in the DDA forums - it varies a bit, but not much, and mostly builds on what has already been said. It also contains several spoilers, but here it is:
Some of us have read Whale's canon list, which has a lot of details I won't go into here. But I'd like to focus on exactly how the apocalypse happened, from the point of view of a normal civilian in New England:
Day -3: The nation goes to high alert. Reasons are sketchy. Rumours of an imminent attack abound. News media briefly brings up stories of a few drug fueled maniacs attacking civilians, but they are quickly buried under speculation on a possible invasion.
Day -2:
Speculation continues. There are rumours of a new street drug turning people into violent, brutal killers. There are sightings of strange, large, aggressive insects in the woods, but no official confirmation.
Day -1:
There are suspected reports that the killers may be under the influence of a biological attack slowly creeping it's way through the population, turning people into killers. Others dismiss these rumours as nonsense. The government refuses to comment, saying they are investigating and looking into things.
A portal opens in a remote field in, but the government responds quickly and suppresses information about the event.
Day 0:
The bombs drop. Many cities in New England are hit with high explosives. The klaxons that are supposed to warn of an attack never sound, and the strikes appear to be precisely targeted, though there isn't much special about the areas they strike at first glance. In reality, several of the smaller labs that scatter the area providing relays for the primary dimensional manipulation experiments have experience gateways opening in the immediate area, and their machinery refusing to shut down. Those inside have quickly been overwhelmed by extradimensional horrors. The strikes are against labs that have been overrun, or where the resonator equipment is still functioning, and locations believed to have opened portals. It is hoped the high energy yield of high explosives will collapse the portals, or at least destroy the equipment that makes it possible for them to open. In many cases, this works. In others, the strikes make things worse, changing how the portals work instead.
From several of these strikes, the portals absorb the energy and create a larger, fluctuating rift that allows noxious alien chemicals to billow forth in huge plumes. Those in the immediate area have their skin blister, boil, and slough off, and the chemicals quickly enter the atmosphere.
Other areas have their portals turn into blistering infernos, or areas of extreme cold. It is hypothesized by scientists in-the-know that disrupting the lab network has caused permanent dimensional weakness and expanded our susceptibility not only to connections to the subprime plane, but also a number of neighboring planes. This is all fairly meaningless to the average citizen, who is mostly under the impression that life has turned into hell-on-earth.
Additional portals begin appearing around the world, even as the number of new portals in New England steadily dwindles.
Creatures from the surviving portals, those expanded or missed by the bombs, begin venturing forth and slaughtering those they find. One portal expands violently, causing an explosion that erupts a cloud of spores into the air before closing permanently. These spores begin floating on the breeze, and several eventually find a home.
Finally, the klaxons sound, and a biological attack is announced. There is utter chaos, and many mixed messages. The military is deployed to try to control the situation, and martial law is declared. It is hard to ignore now that the dead rise again short afterward, and abominations roam the streets. Many head to evac shelters and bunkers, others try to lock their doors and stay in their homes. There is a huge migration of vehicles heading west.
As day turns to night, the first caustic rain occurs, returning the alien chemicals to earth at great distances even as they continue to pour from the portal rifts. The deadly rain lasts for almost 9 hours, killing most of those caught outside while evacuating.
Day 1:
Most of the monsters die off, seemingly only able to operate within a certain distance of the portals, before the effects of the prime plane begin causing severe degeneration. Unfortunately, the damage has largely been done. Worse, a few find the new plane perfectly comfortable, and begin to expand aggressively, while others such as the ooze that causes the zombies adapt to find forms that will let them expand beyond the range of their connection to another plane.
Those who are killed by monsters, rain, or any of the other new threats to appear raise as zombies, and even zombies that are killed with bullets or normal weapons will raise again soon afterwards.
In an effort to combat the new threats of both large amounts of undead humans and various otherworldly horrors, the automated security forces are quickly reprogrammed to behave much more aggressively, killing threats instead of just trying to arrest them if they were supposed to, and adopting a much wider view of what constitutes a threat. Unfortunately, this backfires - they still fail to identify the undead as humans, and continue to ignore them since animals can't be criminals, but the new settings lead them to turn on the human "trespassers" running the security network, and the networked security systems identify almost any living human as a hostile threat to be eliminated. The larger network itself collapses shortly afterwards, leaving no way to adjust the settings back, as each security unit - police station, military turret, autonomous hunter vehicle - reverts to independent control operating under the last instructions received
The nation declares a national disaster, and FEMA camps are setup and a large-scale evacuation begins, even as reports of portals opening in other locations begin to reach official ears. The evacuation camps prove to be deathtraps, attracting the attention of deadly creatures and the security forces who view them as "illegal homeless encampments".
Day 2:
National support is pulled out of New England, and the area is declared a lost cause as efforts intensify to control the new appearances of portals throughout the rest of the US.
Acid rain occurs intermittently. The acid breaks down fairly quickly after it falls to earth, but several portals remain open that pour the caustic material into the sky. Like a volcano, some portals remain quiet for a while before letting forth a huge belch of a toxic cloud, while others release a steady, uncomfortable trickle.
The first fungaloids are encountered by a military patrol, who are quickly overwhelmed. They report the presence of several large fungal growths, almost human height, before the fight begins. By the end of the day, the spires will be significantly larger, and fungaloids will begin to roam in the area.
Triffids, strange flower-like creatures that seem to build upon the existing root networks in local forests, are seen growing in multiple places, having been dropped by Triffid seed-runners that escaped through one or more portals. They quickly attune to the new landscape, and many regions of forest begin slowly converting into triffid groves.
The giant insects begin spread as well, serving as a useful vessel for the goo, but maintaining many of their primitive impulses. Construction begins on enormous hives, dens, and colonies.