What event is considered to be of greatest importance in The Watch That Ends the Night?
Are you refering to the historical events that form a backdrop for the events in this novel? If this is so, I would actually argue that there are two major historical events that you need to think about that are both of equal importance.
The first is of course the Great Depression that occurred throughout the 1930s. The reality of this incredibly important period of history impacts the characters, and George is a perfect example of this. Note how even though he has completed his degree, he is unable to find appropriate work and is forced to start teaching in a private school. This in turn means that he is in sympathy with other disaffected youths so that when he meets the group of Socialists he is already sympathetic to their ideas.
The other event of massive historical importance is the Spanish Civil War. Although this was geographically very distant from the main action in Canada, nonetheless it was historically an event that caused socialist groups around the world to unite, and a number of Canadians did actually travel over to spain to fight for the cause of socialism against fascism. The character of Jerome in this novel acts as one of many individuals who give up a comfortable and successful life in Canada to travel to Spain and fight fascism.