Pompeii is such a treasure and still has so much to teach us about the way people lived during the 1st Century. As such, it is an active hive of new study and preservation work to ensure that what has already been discovered remains safe for future generations to visit and enjoy. Therefore, on any given day some of the streets, houses and other buildings are closed to the public – in some instances for months at a time. In order to best plan your trip and avoid disappointment my tip is arrive in the area the night before you want to visit Pompeii and stay locally.
That way you can visit the gate the evening before your visit and get the map. While there ask the guys at the ticket desk to mark the key areas/buildings that will not be open to the public the next day. That way you can then plan your visit over dinner and avoid planning visits to areas that will be inaccessible. Be warned you will get a frosty reception if you try this before about 4pm as they will still be busy still selling tickets to the people entering Pompeii that day!