FAQ

While the film itself doesn't make the cause clear, there are a few hints given: the light grows brighter and harsher as the deadline approaches; it is fully light, even during what should be midnight in Toronto; and the precise time of the world's end has been known several months in advance.

So that makes the light seen flaring in the sky _not_ the sun, but something else, something that has been visible for some time (One of the characters mentions how he misses the darkness). It is unlikely to be a comet (comets aren't that bright, particularly approaching the sun and in the earth's shadow; also, the impact of even a large comet would not immediately cause the extinction of life on earth) and more likely to be a long, intense gamma-ray burst or a supernova from another star. The evidence for both could be detected before the radiation wavefront hit us, and the time it would take to reach us could be calculated with fairly high precision (since the shockwave moves less than the speed of light).

The good news is that such events, while they do occur, are unlikely - to be lethal to life on earth, a gamma-ray burst is thought to require a source within 3,000 light years of us, and there are no candidate stars for such an event in that vicinity. A supernova is somewhat more likely, but still at (literally) astronomical odds: the white dwarf star IK Pegasi is the closest candidate for going nova, at 150 light years away, and will likely produce a Type 1a supernova, the most dangerous for life on Earth: but it is currently thought it will require several million more years for IK Pegasi to gain enough mass before it goes critical.

It's also possible, the Earth has been set on a colision couse with a planet that has tremendous reflective properties making it act almost like a second sun in its brightness or the earth is falling into the sun or the sun has started expaning into a red giant and is about to engulf the earth. those are all possible although very unlikey and would require enormous suspension of disbelief.

A few critics asked this question, but the dialogue at the dinnertable scene indicates major panic has already played out.