Which day of the week is the least expensive to fly? [OC]

I think we’ve all heard that it is cheaper to fly on some days than on other days.  I have proof!  I’m pricing far out in advance a 7 day vacation to Lisbon for my wife and I (by “7 days” I mean we could depart on a Monday and return the following Monday, for example).  We are going to spend my Delta SkyMiles to pay for it, and we’re going to sit in the posh lay-flat seats up front.  Those seats are very expensive but hey, I want to spoil her (while still finding the cheapest way to do that).  On Delta’s website, you can look at pricing about 11 months out (as I type this on Nov 12 2025, the last departure date they’ll price is Oct 2 2026).  The price fluctuates dramatically over the week, and I got to wondering: which individual departure day-of-week is best?  That led to the red and green graph where it turns out, it is usually Tuesday, with Wednesday and Thursday right behind.  In 21 weeks out of 44*, the Tuesday price is the best price, and in 14 other weeks it is either second or third best; only once is it worst and only 5 times is it in the worst 3.  Friday and Sunday are NEVER the best price, and there is one random Monday with a best price (Jan 19) and one random Saturday (May 23)

Then I wondered “Ok, if Tuesday is the best price most often, but still less than half, when ELSE is the best price?”  That led to the rainbow graph, which other day of the week has a better price and how often?  (he "SELF" bars are how many days that day-of-week was the cheapest within the +/- 3 days surrounding it) A bit of a surprise in that result: there are 12 Tuesdays where the previous Saturday is actually better (but not best)?

Bottom line for our trip: there are exactly 39 departure dates coming up for which I have enough miles to book them, and the pattern holds for that subset: more than half of them are in the Tuesday-Thursday window (9 Tuesdays, 9 Wednesdays, and 6 Thursdays.)

*44 weeks, because I am excluding the two weeks that begin tomorrow with their ridiculously high ‘last minute’ prices

Data source: Delta's website

Tool: Google Sheets