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!
The truth is, there is no single cheapest day of the week to book a flight.
First, no two routes are the same. A number of factors, including demand, seasonality, and competition, affect how far in advance you should book your specific trip and what day will be cheapest.
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.