That's a great question. It's one of the most popular questions we get at Hopper. It's also one of the questions that always seems to be answered wrong.
The truth is, there is no single cheapest day of the week to book a flight.
First, no two routes are the same, and any advice you've heard about always booking on a Tuesday or always booking 60 days in advance? It's baloney. 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.
This is why we encourage users to watch trips with Hopper. That way, we can notify you when prices fall and provide you with an expert prediction about how much you should pay.