The cheapest days to fly are, on most routes, the ones nobody wants: Tuesday and Wednesday. A flight that leaves midweek tends to come in under the same flight on a Friday or Saturday, because that's when leisure travelers crowd in and the fare buckets fill up. That part of the folklore holds up better than the Tuesday-midnight booking myth does. The catch is that the gap is usually smaller, and more uneven, than the advice lets on.
How big the gap actually is
When we watch a route across different departure days, the midweek discount is real but modest on most trips. Shift a domestic hop from Saturday to Tuesday and you might save a little; it's rarely the half-price windfall the headlines imply. The weekend premium gets fatter on routes where almost everyone is flying for a weekend: short leisure city-breaks, Friday-night escapes, Sunday-night returns. It nearly vanishes on routes where the traffic is mixed or mostly business, because those seats sell evenly all week.
So the honest answer to "is midweek worth it" depends entirely on the trip. A long-haul flight you're taking once a year, where the midweek saving is a rounding error against the total, is not worth bending your whole itinerary around. A frequent short route where Tuesday consistently runs cheaper, and you can move a meeting or take a day off, is exactly where the rule earns its keep.
When it's worth rearranging
The thing to weigh is what the change actually costs you. Flying out Tuesday instead of Saturday can mean burning two extra vacation days, or arriving the night before an event you could've made same-day. If the day shift saves you forty dollars and costs you a day of leave, you've made a bad trade, same as paying for a layover that isn't worth the hours.
There's also a moving-target problem. The cheapest day on your route this month isn't fixed; it drifts with demand and the calendar, and a single fare can swing more in a week than the midweek discount is worth. So we don't hard-code a "fly Tuesday" rule into the daily email. We expand every saved route by a few days in each direction and price the real combinations, then send you the three best. Some mornings the cheapest one is a Wednesday. Sometimes it's a Saturday that happened to dip. We'd rather show you which day is actually cheap for your trip than tell you which day is supposed to be.