The very last week of January & first week of February. Very low crowds but the weather can be downright cool (freeze warnings at night). OR, first 2 weeks of December. Weather is usually still pretty good.
However, our favorite time of year is mid-October. Halloween Party, low crowds usually but watch for some schools that have a break Columbus week, and Food & Wine Fest. 2nd favorite time of year is right around Easter (April). yes, the crowds can be big, but the parks have long, long hours so you can either go early or late, and the weather is perfect. Flower & Garden festival and perfect weather.