IME, any time I think I might have an empty middle next to me--including flights which had been selling all Y buckets 9s across the board, and which had sparsely-populated seat maps a couple of days out--I go to do OLCI and get prompted to provide a bid for VDBing. I don't know if it's the result of irrops on prior flights or what, but often by the time I get to the gate, the Y seats have all been assigned where just 36 hours earlier it looked likely we'd go out with empties, and the GAs are announcing a "completely full flight today."

Honestly, I find Tuesdays the most likely day to yield empty seats, because it's neither a big business or big leisure travel day. All the travel bloggers preaching off-peak travel seem to have made Wednesdays busier, JMHO on the routes I travel, though Wednesdays are good as well. Monday, Thursday evening, and Friday are all business travel heavy days IME; Friday, Saturday, and Sunday are big for leisure travel.