Late summer and fall are the best time to improve a lawn. Fertilizing in early September gives a big benefit to cool-season grasses like bluegrass. Specifically, slightly thin lawns will begin to fill in, density of the turf will improve, and roots will store more carbohydrates away to help improve winter survival.
Fertilizer used in the fall is typically close to a 4-1-2 or 4-0-2 ratio between nitrogen, phosphorus and potassium. Examples of this might be a 24-5-10 or a 30-0-5. Do not worry about finding these exact numbers; find something close and affordable to your budget.