I do not know any specifics about your problem - how many bare spots, how large are the bare spots, the condition of the rest of lawn, etc.  So, while the following information is general in nature, it can provide you with some guidance.

Late summer to early fall usually is the best time to establish a new lawn from seed. New seedlings are normally more successful at this time of year than in the spring because of reduced weed competition, and the new grass will have two cool growing seasons (fall and spring) before it encounters its first period of heat stress. Also, soils may be too wet for good seedbed preparation in the spring. 

Sometimes conditions may require a spring seeding.  The article from Buckeye Yard and Garden provides specific information on how to do this.  Spring Lawn Seeding | BYGL

Lawn Care - Franklin Soil and Water Conservation District also has information on lawn care and soil testing.

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