My guess is that, you'll Know when N. doesn't want to 'rise and shine', one morning, and then maybe the next one, too.
Nobody (I've encountered) knows any 'rules' that make picking Which Day?? any easier.

(Sierra made her choice evident: I had taken her in, thinking This was the day.)
Her forceful rejection of the efforts to insert a *catheter into front leg, showing more energy in that YOWL than all week
-- made it a great relief to say, Stop! she Isn't Ready. She got 3 weeks of Life #10, via that spirited NO!!

Luck,

I.


* fwiw Believe now that Na-pentothal injection beats the "2-stage" process used by many vets. As I have had that (same stuff) used, I Know how effective/instant and painless a soporific it is.
The catheters (for cats fershure) entail just the kind of handling + initial pain of catheter insertion ... which makes them dread a vet visit, anyway. Doubt that dogs are very different. Sorry..