When you overseed a lawn in spring, you’ll then have to keep the area constantly moist; the same as when you start a lawn on bare ground.  If this isn’t done, most of the seed will never sprout.  The best time to over seed is late in the fall like around Thanksgiving or later.  This is called ‘dormant overseeding”  this allows the seed to get pushed down into the turf and get soaked by rain and snow over winter so it sprouts naturally in spring.  If you are wanting to repair bare areas in spring, wait until earlt May when it is warmer so the seed sprouts quickly.   Keep the see moist constantly intil it is well established.