Don't know where you are, but for most areas of the US with cool season
grasses, it's way past optimal time for seeding. The best time is
early Sept, second best early spring. Now you have hot weather on the
way, which makes it much harder.
Also, I agree with Peter, that unless you really have a thatch problem,
de-thatching is necessary. You can overseed by cutting it short and
using a slice seeder, which is a lot less work.