If all you're concerned with is crowds, check out touring plans dot com (if you sign up, you get access to their crowd calendar projections that currently run through the end of this year). Or sign up for tour guide mike dot com. He has advice on that subject as well.
Generally speaking, though, if you go this time of year (avoiding US holidays such as MLK day, President's day, Easter, etc.), you'll find low crowds, but there will be a large number of attractions down for refurbishment.
Another option is to go in late April/early May, which has moderate crowds and usually less attractions down for refurbishment.
The least crowds you're likely to find will be in September (after Labor Day) and early October. But it's warmer then (it can be downright hot in September, not to mention humid and rainy, and not to mention the possible hurricanes). On the other hand, you'll find very few attractions down for maintenance, if any.
The final opportunity comes after Thanksgiving weekend. Up until a week or two before Christmas, crowds are moderate and there's usually very few attractions down for maintenance. But the weather is a big variable. It can be cool one day, and hot the next. You never know what you'll get.
If you can, avoid going any time children are not in school. So, summer vacation, Christmas vacation, spring break, etc. Crowds are at their maximums during those times.
But, it's possible to have fun any time you go, if you plan ahead. Keep reading this site, sign up for tour guide mike and/or touring plans, buy yourself a guidebook such as the Unofficial Guide and/or the Passporter, and you'll be in good shape!