We go every year in early February. It is awesome!! No crowds, no lines, it's wonderful!! It is cooler at night, and one year, it was downright cold, but during the day, it's great. We usually wear shorts and just take a sweatshirt or sweater for night. My oldest has just started kindergarten this year, so I guess we're going to have to give up our February vacations, and I'm really going to miss them!!
We did go one year closer to the end of the second week. We were in Magic Kingdom on the Friday that started Presiden'ts Day weekend - didn't think about it when we were making our plans. Even still, it wasn't that bad. Crowds were definitely picking up that day, and I'm sure Saturday was crazy, but if you aim to be out the Thursday before - you'll have a great time!!