The time of year can drastically alter your freight rates. Since supply and demand is key, at a time of year when demand is down, rates are generally cheaper. This is especially true in the construction arena, and, especially in areas where the weather is cold in the winter. For example, in Chicago, during peak construction season, every construction truck is running hard, six days a week. But, in the winter time, most of them are parked. So, if you have a mountain of sand that you need to move, January would be a great time to negotiate a good rate to have it done.
On the other hand, if you must move it during peak season, expect to pay full rate.