The inexpensive day to book low-fare flight tickets on United Airlines is Tuesday, as per the fare trends. Like other U.S airlines, United starts its flash sale either on late Monday or early Tuesday. It reduces the ticket fares to most of the popular routes, and flyers can grab great discounts on Tuesdays. Moreover, the ideal time to find the cheapest flight fares is between 6:00 am to noon. The tickets may rise during the day and go down in the evening slot.