It depends more on where you live than what zone you live in. Can you share that with us?

How much shade hits the thin areas? If you're trying to repair a shady area you need a fine fescue instead of KBG.

The cottage is in Wells Maine in southern coastal Maine; approximately 2 miles from the ocean. The area faces east and gets at least 6 to 8 hours of sun per day.

If your prevailing wind is from the shore, then I'd say any time would be good. If your prevailing wind is from New Hampshire, then I'd wait until the end of this month. Earlier is better as long as you don't have temps in the 90s day after day.

Is there any special prep work required? i.e. cut grass in area to a shorter length, rough up the area???

Thanks