What day of the week is flight travel lightest?

Generally Tuesday through Thursday yields the lowest ticket prices, which I can only assume relates to volume (you want to fill the plane). For overseas flights though the dynamics change, seem to have been on very full flights most of the time, and early June is getting into peak season.

I have not flown Standby, but it would seem there is significant risk, unless you have a healthy buffer of days and flights before you absolutely need to be there. I don't get the idea of flying standby for a normal passenger, if you are an airline employee, or other with benefits, then that changes the calculus more.

You can certainly start doing trial bookings in the weeks before for flights, and see what is filling up, or not. Does it cost to be putr on a standby list? Cost to remove yourself? My thought is if you see a flight that is just not filling, that is the one you want. But like placing any wager, do not bet more in non-refundable reservations than you are willing and able to lose.

https://thevacationer.com/best-and-worst-days-to-fly/#The_Best_Days_to_Fly

This site echos what Paul said.

If you’re flying standby I’m assuming you’re flying on some kind of airline employee benefit because that’s about the only way standby exists now and whoever your sponsor is with the airline should be the person answering this question. they can see a lot more than anyone on here is going to be able to see to help you

Your thread from December got a lot of answers although that was presented as a slightly different situation.

https://community.ricksteves.com/travel-forum/transportation/slowest-day-of-the-week-for-travel

My answer is still the same. Since Covid, every flight I've been on (Delta) has been completely full no matter what day of the week.

I've been on some very thin international flights on Delta, but that was during the winter. All my summer flights have been very full. As noted above, Tuesday through Thursday tends to correspond with lowest fares, which relates to number of people traveling on those days.

If you're flying standby, a lot depends on what kind of standby. If it's because you received it from a friend or relative who works for the airline, a lot will depend on their status. Seniority counts with standby, and also your relationship.

I flew standby for years off and on as my brother was a Delta pilot, and sometimes you get on and sometimes you don't. And as Pam said, it's even worse post-Covid. The days of flying on half-empty flights are long gone. You really won't know until you're there, so I would count on making sure that you have enough time to hang out at the airport.

And I would not pre-book any reservations unless they are completely refundable.

As former business treveler I would add times.

Tuesday afternoon
Wednesday full day
Thursday morning
Saturday morning and evening
Sunday morning