Short answer: departure day does affect price, and mid-week departures are often cheaper than Friday or Sunday ones. But the effect is smaller and less reliable than the advice suggests, and it is frequently confused with a separate claim about which day you book.
Two different things get muddled here. Keep them apart.
Departure day: a real, explainable effect
Fridays and Sundays carry leisure travellers going away and coming back. Monday mornings and Thursday evenings carry business travellers. Tuesday and Wednesday sit between those peaks on many routes, so seats are easier to fill and pricing reflects that.
This is a genuine pattern with an obvious mechanism. It is also a tendency, not a rule — it varies by route, season and carrier, and on some routes it disappears entirely.
Booking day: much weaker
The claim that booking on a particular weekday gets you a better fare is a different and far shakier one. Fares are repriced continuously by automated systems responding to demand and competitor pricing. Treating the day you happen to open your laptop as a lever is not supported.
Why the advice persists
Because it is easy to repeat and impossible to disprove from a single booking. If you book on a Tuesday and get a good fare, the rule appears to work. There’s no counterfactual to compare against — you never see what the same seat would have cost on Thursday.
That’s the real difficulty with all fare folklore: a single traveller has a sample size of one and no control group.
What to do instead
Flexibility is worth more than day-of-week rules. If you can move a departure by a day or two, checking those neighbouring dates will usually tell you more than any rule of thumb.
And when you can’t be flexible, the question stops being “which day is cheapest” and becomes “is this price good for this route right now” — which means judging a fare against that route’s own history rather than against a general rule.