The best time to mow is when grass is dry. When moisture from rain or the morning dew weighs grass down, the blades bend, making a straight cut difficult. You also can slip on wet grass, and the clippings tend to clump and not spread evenly. In addition, disease can spread quickly when you mow wet grass. That's because a wet, freshly cut grass blade is more susceptible to disease-carrying organisms [source: Boyd].
However, there might be times when you have to mow wet grass, especially when it rains a lot. Experts at Kansas State University say it is better to mow wet grass than letting grass grow too tall and then cutting it when it is dry. Conversely, don't mow grass during a drought or periods of dry spells because mowing stresses the turf [sources: Boyd; Lowe].