We've gone twice in January and loved it both times. It usually doesn't get above 75 degrees. Some of the rides are closed but the crowds are very small. You can look on the DL website to see the official list of what will be closed.

This year, we went the first week of February which is now the park's annual passholder month. The crowds were bigger than when we've gone in January, but still much smaller than you would get in the summer.