Hi simonruss
New York at christmas time is really great!.............they usually start putting decorations up just after Thanksgiving day, which is always the fourth thursday in November, sometimes slightly earlier, and they look spectacular once done, especially around Central Park near the ice skating rink ( or in my case......ice fall!........always on the floor!) .........so safe to say most of the shops and streets all decorated and done by end of November.
hope this helps you.
All the best
yogi bear/ theEqualizer