Source: Natalie B. Compton, Washington Post ( Paid Registration )

Google Flights data showed prices for domestic flights have usually been their lowest between 21 and 60 days out. Those prices tend to bottom out around 44 days before takeoff. 

Look for flights that depart in the middle of the week instead of weekends. Flights early in the week are 12 percent cheaper than weekend ones, and Sundays are particularly pricey.

Read Full Article: The best times to find cheap flights