Whether there are slow times or not will depend a lot on your motor carrier, equipment and credentials. A team that is configured one way may be very busy at the same time a team with another carrier and different equipment and credentials is complaining about a slow week or month. One carrier may have a large account with a shipper who ships many loads in July. Another carrier may peak in a different month.
Note also that where slow times are said to exist, it might be very busy for a team that remains in service. For example, many expediters take time off for holidays. That reduces the supply of available trucks for the smaller number of loads that are then available. A team may very well score a sweet load, not because the times were busy but because they remained in service when many others took time off because it was a so-called slow time.
Instead of thinking about how to play the slow times, which is difficult to do because the picture is so big and there are so many variables, think about the kind of schedule you wish to have and then look for the carrier, equipment and credentials that will maximize your opportunities in the times you will be in service.