There is no CHEAP day to book a cheapflight. Focus on the day you are flying rather than the day you are buying the ticket. What we can suggest are the most awkward times. The more red-eye or late the flight, usually the cheaper it is.
The cheapest days to fly are usually Tuesdays and Thursdays. Avoid Sunday, Monday and Fridays. They seem to be the HOTSPOTS for travellers, especially locally – no super cheap flights on those days.
The recommended date to book a flight to get the most common cheap flight is 6 weeks prior to take-off. Airlines have checked and the average suggests that is the sweet-spot. 2 reasons – affordability and availability.