Mid-August to mid-September is the best time to seed cool-season grasses. However, sometimes turf is damaged throughout the winter or the previous year and must be seeded in the spring. Spring seeding is difficult so there are a few helpful hints to ensure the most successful outcome.
Seeding should take place early in the spring (early April) to decrease weed competition. Prior to seeding, the soil should be disturbed by raking to improve seed-soil contact. Once soil temperatures climb above 50-55 degrees Fahrenheit, germination will occur in the soil and the area must be irrigated often. For newly-established lawns, the soil should be kept fairly moist throughout the entire summer. Finally, do not use broadleaf herbicides in the area until the new turf has been mowed at least three times (for preemergence herbicides, the turf should be mowed four to eight times).
More information is available at: http://www.agry.purdue.edu/turf/pubs/ay-20.pdf