People will tell you that you'll just know. They mean well and it's useless advice.
What actually happens is you lie awake doing math on a creature who can't tell you anything. She ate this morning but not last night. He got up for the door but not for the walk. Is that a good day? Was yesterday a bad one? Am I keeping her alive because she's still in there, or because I can't do it yet?
That's the real question, and it deserves a real answer instead of a feeling you're supposed to magically have.
So here's the thing nobody says out loud: there's usually no perfect day. There's a window. Somewhere on one side you're taking time your pet still wanted, and somewhere on the other side they're suffering while you catch up emotionally. Most vets will tell you privately that a week early is far kinder than a day late.