Saturday afternoons/Sunday mornings in business markets would always be better bets to clear an upgrade due to lack of business travel. In leisure markets though, the opposite might be a decent generalization. Leisure markets I'd be more inclined to roll the dice midweek but if it's a "leisure" market with decent paid F/J demand (think cruise ports or anywhere rich people go) or convention town (LAS/ORL), then all bets are off.

(LAS is actually a strange market. It has surprisingly strong paid F demand)

I used to fly DFW-ATL quite often and spread the love around between AA and DL. I was fascinated by the complete imbalance of upgrade lists. DFW origin was extremely heavy on AA Monday mornings while ATL origin was extremely heavy on DL Monday mornings. Flip that around for the Thursday afternoon returns.

If you have the flexibility to wait until closer in to book, or at least the flexibility to leverage no change fees or free award res redeposits, you can always change to flights that give you a better shot. But, again, mind the advance booking curve. This is a sweeping generalization but business markets take their bookings close in while leisure markets take their bookings far out.

Thus, last summer, when I saw a Saturday night DFW-ALB flight was being operated by an A321 rather than A320, I figured the nearly empty seat map a couple weeks out told me what I needed to know about whether to spend money buying an upgrade/upfare to F. I did not. My upgrade cleared. The fun anecdote though was that I think the flight went out with 11 pax in F. Guess who the one pax to NOT have an empty F seat next to him was?