Comets hitting the Earth is the other favorite topic for sci-fi writers to hypothesize about.
The best book about a comet hitting the Earth is Lucifer s Hammer, which accurately describes how a comet might split into pieces as it enters the Earth s atmosphere destroying different parts of the Earth. Fortunately a devastation of that nature is expected once every 60 million years. There is no best time of year to look for comets. In fact, there is no best place to look either. I personally find it amazing that comets are so often discovered simultaneously within a few hours of each other considering how difficult they are to find.