Let's start with the date. Amazingly, Holden's story takes place over only three days, from Saturday afternoon to Monday around 1pm. Any big fans of the novel already know how to figure out the exact year of the story – check out Chapter Five when Holden's talking about Allie's baseball mitt. He says Allie died on July 18, 1946 when Allie was eleven and Holden was thirteen. Back in Chapter Two, Holden mentioned that he's seventeen now (as he's telling us the story) and was sixteen "last year around Christmas" when he left Pencey and bummed around the city for a while. So the year of the December New York City escapades is either 1948 or 1949, depending on 1) when Holden's birthday falls and 2) what the exact date is of his story-telling. It follows that the year of seventeen-year-old Holden telling us his story is either 1949 or 1950.
What's the significance of 1948/1949/1950? It's post-World War II. Holden talks about the war (and the effect it's had on his brother D.B.) with a slightly detached air. He mentions the Atomic bomb, which the U.S. busted out in August of 1945, four to five years earlier than Holden's narration. You can see the dropping of the bomb as a sort of a nation-wide "loss of innocence" if you were so inclined.
Additionally, Holden is getting at some of the general feelings of isolation and disillusionment of his generation. He also may be rebelling against the growing conformism and consumerism of America. So while we can still understand (and be enamored of) Holden today, we have to read The Catcher in the Rye with a grain of historical perspective salt.
What about geographical setting? We go from Pencey Prep – land of the phonies – to New York City, land of the…phonies. Holden thinks that by switching location, he can escape the people and attitudes he dislikes. But his internally-generated isolation goes with him.