From paragraph 21:
Fenrir is out there ready to tear us to pieces at the fall of night.
With some Internet research, I'm finding that this particular phrasing is said to be associated with either colloquial Irish English, or with Sanskrit [specifically in the Quran]. I cannot recall there being any mention of the ethnic roots or the religious beliefs of either Robert or his brother Pete throughout the novel. The last name Gottschalk appears to be a German name that translates to "servant of God", but Islam has never been a major religion in Germany. Therefore the use of "at the fall of night" here seems to be awkward; the more appropriate phrase here would be "at nightfall".