I prefer the last week of January. You'll have to pack all kinds of clothes; shorts, tees, sweatshirts and a jacket. Our last trip in January it went down to a record breaking 28 degrees one night! The next morning we were waiting at the bus stop and the CM's told us it was 36 degrees! Other than 2 cold days, the other days were in the upper 60's and low 70's.
Even though the weather can be unpredictable it's crowd free with no humidity! Late January, early February is my favorite time to go!