BOS-ORD - 6/12/24
T-48
Fare: S
Booked 2/26/24

I was originally booked on PIT-EWR-IAD this morning. The first segment was a guaranteed CPU, with me #1 on the list and 9 available seats (the 2nd unlikely, with me #4 with 2 available).

I changed to non-stop PIT-IAD, saw I was #4 with 4 available, then checked refaring... $49 to refare to P. Took it, and noticed they only cleared 2 upgrades (albeit they could have cleared a GS or two into PN). Honestly, it's a short flight to begin with, so Y is no big deal, but I was amazed they were still selling P that close to the flight and was happy to buy up at that price point when it was unlikely I would have cleared.

BOS-ORD - 7/4/24
T-36
Fare: K
Booked 3/22/24

ORF-EWR - 7/21/24
T-2hrs
Fare: E
Booked 5/4/24

My spouse was upgraded at the gate as a Silver, separate res.

DEN-EWR 8/7/24
Fare S booked on 4/5/24
Numbers 14 and 15 on upgrade list, F booked and checked in Full
We board after a slight delay in E+
ATC won't allow departure at 1:25 suggests new wheels up of 3:06
everyone disembarks to the gate.
Reboard at ~ 2:10
GA gives us new BPs for 1B and 1E, woman behind us had been upgraded to 1A and she offers to switch to 1E a very nice gesture.
Wheels up pretty much at 3:06

YYZ-EWR 7AUG24
T-36 before departure as 1K
Fare: K
Booked 1 month or so ahead

Plane was originally scheduled as E-170 with 8F seats, but switched to E-175 with 12F. At check-in, it showed 4 booked and 2 others on UG list. But the flight went out full in F somehow. This was also my 1st CPU this year after dying as #8 or higher on the UG list on other flights.

IAD-MSP Sunday 11 Aug 24 E-175
75 minutes before departure as a Silver
4/12 F were booked with 5 on the upgrade list, and my lowly Silver self at the top. Was this way for days but didn't clear until right before. Non-revs filled it out.
First domestic trip as a silver. 100% upgrade success! :D

EWR-MKE - 8/29/24
T-96
Fare: U
Booked 6/22/24

Are Tuesdays and Saturdays generally the best days to fly if you care about upgrades? I know it’s dependent on many other factors as well but I have been booking on Wednesdays when I want to clear upgrades.

Tuesdays, Wednesdays, and Saturdays are the lowest travel days for most destinations. However, airlines take that into account to some extent, and you’ll often see reduced schedules on those days (particularly Saturday).

I've had greatest success on routes that are not heavy on OPM flyers, and midweek.
But it really is YMMV.

Last week on a 7am EWR-YYZ on a Wednesday, the plane was full of suits, and I barely cleared my CPU as 1K.
The preceeding flight CPT-EWR, no issues from a W fare using pluspoints, there were hardly any OPM flyers on the plane on this route
I've also cleared SFO-SIN on a K fare without issues on a Monday.
So who knows.

How can you tell which routes are OPM? Is there a dedicated FlyerTalk thread for this I can read? My travel schedule is very flexible so I’m ok flying midweek to get an upgrade on pluspoints instead of paying real money.

You cant. But think about it. OPM flyers internationally tend to fly at the beginning/end of week. Not many fly to work on a Wednesday.
Some destinations like PVG, LHR, BRU, DXB etc tend to be OPM heavy. Some others like CPT, CHC, NCE, PMI etc are probably not.
But its just a general pattern, and not exact science

Good to hear - we are not going to requalify this year, and will just keep MM Gold.
Its been very refreshing flying a dozen different airlines for our vacations this year, and not worrying about PQP

Thanks. Interestingly many PVG flights clear on pluspoints and there's OK PZ and IN availability in LHR.

I also think Gold is the most important tier to hit with United. That makes 1 million mile goal very attractive but 2 million less exciting. I find value in 1K but it's more ymmv than Gold. I often feel Gold is too good of a status but some elements are a result of Star Alliance. I wish 1K and Polaris still had separate check-ins in airports.

[My emphasis]

Many still do. I believe it’s hubs and most long-haul outstations (ie I know TPE has it).