Cataclysmic is the describing word for a cataclysm: a cataclysmic flood, a cataclysmic war — sudden, violent, and big enough that life afterwards is not the same. Scientists also use it for huge shifts in the earth or the climate. People stretch it when they want drama: a team might call an 8–0 loss cataclysmic. That stretch is common, and a little over the top; the flood is still hiding in the word. It is a serious, slightly bookish adjective, not the first choice for a merely bad day.