When is best time to overseed
Hennepin County Minnesota overseeding
Late summer/early fall is an ideal time to over seed bare places in your yard. There are two key considerations in doing so: soil preparation and seed selection.
The following sites will provide you some information for prepping your soil:
With respect to seed selection, this will depend upon the amount of sun received by the areas that you will be over seeding. For example, Kentucky Blue Grass does not grow well in shade but requires a fair amount of sun. On the other hand, various fescues do well in shade. See:
Instead of purchasing a grass seed mixture from a big-box store, you may want to buy individual types of grass from local gardening supply greenhouses.