Throughout the Northern Plains and Upper Midwest, the most humid months of the year are July and August. Dew point temperatures are often in the 60s and 70s. There are two reasons for this: geography and agriculture.
The geographical portion is all about location. The source of the most humid air that reaches us in summer is the Gulf of Mexico some 1000 miles to the southeast. As weather systems pass through from the west during spring and summer, producing southeasterly then northwesterly winds, it takes a while for the air in our region to become mostly of Gulf origin. In fall, as the storm track slips southward, northerly winds begin to predominate.
The agricultural factor has to do with the biochemical processes of photosynthesis. Water vapor is released when the plants are bathed in sunlight, and the more robust crops of high summer are more efficient.