When is best time and how to overseed a yard with grass seed?

What is your location? Are you in full sun,shade or both,What kind of grass is there now?

The guideline I was given by the Scott's rep I ran into at my Lowe's was fall Sept 21 and spring March 21. This is for Sussex county Delaware. http://www.plantmaps.com/

I usually do it in the early spring. I watch the weather report. I try to do it, just after a rain but before a big rain. The rain will wash the seed away. Planting in heat, will just burn up the seed. Seed will grow best in the cool spring weather.

..... & birdies looove grass seed, so I always put up string with pieces of fabric strips to blow in the breeze to try & keep them away!