This is a popular (and often wrongly answered!) question. No, Tuesday isn't always the cheapest day to book a flight. No, you shouldn't be searching for flights at midnight!
It's important to note that every airline route is different, so the best time to book your flight may be different. This is why we recommend setting up a trip watch in the Hopper app. Using massive amounts of data, Hopper can make a prediction about when is the best time to book your flight.