We are never told precisely where The Graveyard Book is set. However, we can infer the setting from clues the author leaves us. Here's how:

 

When - There are cars and we find out that Scarlett's mother teaches university courses over the Internet. Therefore, it must take place in modern times. Certainly this millennium (the 2000's).

 

Where - They speak English and all have English-sounding names. Therefore, it must take place somewhere English is spoken. One of the oldest inhabitants in the graveyard is a Roman who mentions how cold it is. Also, Scarlett wears and anorak (a warm jacket), so we're definitely somewhere that has all 4 seasons. Some of the inhabitants also talk about having seen the queen. Given that evidence, and some of the language the author uses, this story takes place in a graveyard in England.

 

Circumstances - We don't have to infer anything for circumstances. We know Bod has been