It all depends on the look you are going for! If you want bright sun and kids playing then anytime between 9-5 will work. If you want sunset photos you need to get them done at least 1.5 hours before sunset for the soft golden hour. IF the day is overcast you'd have to reschedule. While overcast sky can be an asset to photographers in some situations, having no dynamic lighting doesn't give you anything to work with. I personally like when the sun is out because you can backlit people, side light and you can always duck into shade if you really need better control ovwer the light.
Location: Gloucester Massachusetts.