Most people reseed, aerate and fertilize in spring and get it backward. For cool-season lawns, the ideal window is late August through mid-September. Cool-season grass seed germinates best when soil sits in the 50–65 degrees sweet spot. Seeding now gives new grass roughly 45 days before the average first frost to root and get mowed twice before winter — a real head start on dormancy and a jump on next spring.