Slowest day of the week for travel?

Supposedly it's Tuesday and Wednesday so I have read BUT....the last few trips I've done...well actually since Covid (6 international round trips since 2021), even those days the flights were fully booked. IF you want more space, I'd buy up to either Business or whatever your airline calls it. On Delta it's Delta One and there are lines of single seats down either side with 2 seats together in the middle.

Might be good to ask your specific airline.
I've found 2 seats in economy give more room than the business class ones, and were cheaper, but that was a while ago.
I learned the hard way not to choose seats in the back of the plane to get those extra empty seats near me for free, because the turbulance kept me awake all night.
Happy travels...

I typically fly on Tuesday, which seems to offer about the cheapest fare of the week (though sometimes other days will match it). However, I don't remember having a seat empty beside me on the transatlantic leg since I resumed traveling in 2015.

I'd assume examining fares will give you a bit of a clue, because the airline algorithms are designed to yield full flights at the highest fare travelers will accept.

I always tried to fly on Tuesday or Wednesday for long distance hauling. However, lately, and especially since the pandemic, most flights have been very full.

Now I also look at Thursday but do my best to avoid Friday, Sunday and even Monday depending on time of the year.

And just assume between June and September, every day is going to be busy. People are traveling.

It is always a gamble. But, like you, i generally want an empty seat next to me. So far I have done good by

• not booking too far in advance. Maybe 3 months. The difference in price is negligible if at all.
• Picking an aisle seat in the center section of seats, on a row where the aisle seat at the opposite end is already sold. The seats between the two will be the last to sell, cause who wants to sit in the middle.
• Then at check-in I double check and move my seat if I have to.

9 times out of ten I end up with my empty seat. A few times during the final check I have spotted an entire row empty. I move to that row and stretch out across the seats and sleep. I have traveled to europe in every month of the year. Will the plane be empty or full is a crap shoot.

I prefer this over choosing a ticket on day of the week, because I prefer to choose the cheapest tickets I can find, regardless of the day of the week. For that the Google Flights date matrix tool comes in very handy.