The Question and Answer section for The Turn of the Screw is a great resource to ask questions, find answers, and discuss the novel.
This is a haunting story. I think the author wanted the reader to have some freedom of imagination as well as concentrate on flushing out the main characters.
Douglas knew and was in love with the governess as a young man, when she was his sister's governess. This may bias his description of her. He has kept her manuscript in a locked drawer in his home for many decades.
In the prologue, Friends gathered around a fire in a country house outside London on Christmas Eve entertain themselves by telling ghost stories. There does not seem to be a date or place.