Cool-season lawns: why then
Cool-season lawns overseed best in early fall: soil is still warm enough for fast germination, air is cooling, weed pressure is collapsing, and seedlings get fall plus spring to mature before their first summer. The window is timed 45–60 days before the first fall frost so new grass is established before cold shuts growth down. Spring overseeding is the backup — it works, but seedlings then face summer heat and crabgrass pre-emergent conflicts.
Warm-season lawns: why then
Warm-season lawns establish from seed in late spring, once soil has warmed well past the last frost — Bermuda and Zoysia seed germinates reliably only at soil temperatures around 65–70°F. The window is timed 2–4 weeks after the last frost; seeding much later still works but shortens the first growing season. (St. Augustine has no viable seed — it establishes from sod or plugs in the same window.)