Nathia Gali is very crowded during Independence Day of Pakistan ( 14th August) week. All hotels are fully booked and the ones which have space would charge you two to three times more then the normal rate.
People even sleep in mosque on 14th August because no space is available any where else. All consumer food items would be expensive. Restaurants would have no space to handle guest and service would be worst.
Lot of funky and not so nice youngsters would be on roads. They often misbehave with others and often make cheap jokes. My suggestion is to avoid trip to Nathia Gali from 12th to 15th August.