I think around may would be a good time. Reiterating what the other poster said, April and March are the spring break/Easter vacations (dd's spring break is the end of March, so we'll definitely be there.)
As for taking your 6 week old, I know you didn't ask for an opinion about that, but since it was brought up, as long as you think you can do it and your doctor has okayed it then why not. We took DD (2) when she was around 2 months old and it was fine. It was around that same time frame too, so perfect weather and crowds. Then we went again when she was 6 months old.