When flying from Washington, D.C. to American Samoa, the time of day you decide to fly out makes little to no difference towards the overall price you’ll pay on tickets.
When deciding which route to take from Washington, D.C. to American Samoa consider Washington, D.C. Dulles Intl - Pago Pago, Baltimore/Washington - Pago Pago or Washington, D.C. Reagan-National - Pago Pago as they are the most popular.
These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Washington, D.C. Dulles Intl Airport to Pago Pago Airport flight would often be abbreviated as a IAD to PPG flight.