In Cincinnati, the ideal time to seed a lawn is in late summer to early fall: August through September. The soil is warm, hopefully you've addressed any weed issues over the summer and the days are cooler, so not to add unwanted stress to the emerging lawn.
We don't always have the option of dictating when we tackle a landscaping chore. A new home, a recently renovated landscape or perhaps some unfortunate lawn damage may call for addressing the need for new lawn in the spring. When this is the case, we prefer to use sod to establish a new lawn. To prepare for sod, we never till the soil. Instead, we lightly scarify the surface after all vegetation is killed. Low areas are filled with fresh topsoil, and we may skim all areas with topsoil if desired (1-2"). The sod's root system, which is already in place, will eliminate the need for excessive weed control measures. Nutrition and watering is the key when installing sod in the spring. Deep watering is preferred until the roots are established.