I have 6 large parrots and it appears that the only time they really scream is at dinner time, early in the morning and at dusk. These are normal time frames for parrots. The are a flock bird and it is their way of calling in their flock mates. They also eat together just like a human family would.
The main reason that a parrot will scream other than that is because they are bored. Make sure that they have plenty of toys and attention so they feel like part of the family. These birds need to be around people. It is very important to keep them socialized or they can become mean and aggresive. They may also attach themself to just one person and get very upset and mean if not socialized correctly