But this is very old and it's not too accurate anymore. So take this with a BIG grain of salt. For example, October was crazy for us this year and Early February was dead, which is not what this graph seems to indicate.
October (and overall the whole year) has gotten much more crowded in recent years past. I also agree with Hydroguy's post.
Here's what I would say:
Lowest crowd: January (except first week and MLK weekend), early February, First 2 weeks of December
Low crowds: Last week of February, November (except T-Giving and Veteran's Day)
Medium crowds: First 3 weeks of May, All of September, All of October, Thanksgiving
High crowds: Any weekend, President's Week, March and April (except Spring Break), Late May to Late June, Late August, 3rd week of December
Very high crowds: Any holiday weekend (other than Labor Day), 2/3 weeks surrounding Easter (also known as Spring Break), July to late August, Week after NY
Most Crowded: Week between Christmas and New Years.