Our family has traditionally gone to DLR in early/mid Dec every year for several years. It used to be a quiet time to go, but word got out and now many people plan their trips during that time. It's not near as quiet as it used to be, especially with as many local AP holders who love to enjoy the parks during the holiday season and also seek out less crowded times.
In order to mix things up a bit and seek out less crowds, we've gone at alternate times the past couple years. We went the week after Thanksgiving last year, and found it to be relatively quiet with short waits for most rides. However, we also had some rainy weather, which I'm sure helped keep locals away.
Two years ago, we went for the last day of the holiday peak period in January, so we could take advantage of seeing all the holiday stuff, then spent the following three days in the parks. It was also a pretty quiet park during that time, but hit a heat wave of 87 degrees (in January!!).
It's really a crapshoot to find when the crowds will be at their lowest these days. DLR has done an incredible job with marketing the last couple years (anyone else notice a HUGE increase in DLR commercials?). I have found the parks always feel full, even if lines are relatively short. JMO.