My last trip to Denver really showed me this. I always booked flights for a Sunday, thinking it was the cheapest way to end a weekend trip. I was paying close to $400 each time. Then I checked a Wednesday flight on a whim and it was $240. I felt pretty silly for not trying a midweek date sooner.
Wait, but doesn't that depend on where you're flying from? I mean, I've seen the opposite happen sometimes. I booked a Wednesday flight once thinking I was being smart, and it was way more than the weekend price. Maybe airlines are just all over the place now, idk. It feels like the old rules don't really work anymore.