Mid to late August would be perfect since that is when the hatch begins.  Apply again in mid to late May.  After 3 years of this, you’re all set.  The most important thing to remember is that Milky Spore needs to be watered-in immediately after application or it dies and won’t work.  It is also light sensitive (light kills it) so the ideal situation would be to apply your Milky Spore right at dusk and water it in then with an inch of water…you can use a tuna fish or cat food tin to measure the water.  Don’t count on rain to do the job.  Every minute it is exposed and not washed into the soil diminishes the effectiveness.