My wife and I run with a large carrier in a D-unit. Our carrier tells us that July and January are two of the "slower" months. Yet when we review our numbers, we find July to be a great month and January pretty good too. We've concluded that while the months might be slow for our carrier and perhaps for the industry, enough trucks go out of service in those months to leave an ample supply of freight to haul.
Industry-wide, there are definately slow months. Truck-by-truck, freight availability is determined by a host of variables like the amount of time you are in service, they types of loads you are willing and/or able to haul, the kind of truck you have, the carrier you are affiliated with, and your carrier's ability to retain and add customers.
Instead of asking about which months are slow, you might also want to ask, "In the so-called slow months, how can I position myself as an individual expediter to do well?"