It is usually skewed towards night because that is usually when you are in your base the most. The raid cooldown is 46 minutes with a 20% chance. This is known information so there is no need to speculate if it is random.

Originally posted by electricdawn:

One evening, pretty much at nightfall, we had our first "Foul stench" event, and right the next evening, also right around nightfall, our first "The ground shakes".

This should not be possible unless using mods, as the day/night cycle length is 30 minutes. Which would mean the next possible raid would be the second morning after the nightfall raid.