This indeed a hard question but as one of the few who had to make this decstion not to long a go with my Merlin Man. I really do beleive its better to pick the day and make it a good one . That way if possible its a good memory. The day I picked for Merlin(cancer) was a sunny ,very light breeze fairly warm spring day.He got a nice car ride and trip to the bank for a bone and last time visit the drive through gal. The vet has horses and a nice yard with trees , so we sat under the trees watching the horses and just hanging out relaxing. The vet and asst. both just sat for a couple minutes with us talking to and about Merlin , then just started petting his front leg and slipped the needle in . I don't think he even knew it happened . He just slipped in to my lap and was seamingly asleep. Since he had to go it was the best way I could have planned for him.
He was facing all kinds of drugs and horrible things , I didn't think he deserved all that pain just for me to try to hang on to him for my selfishness.
I really miss him espesally since he was my alert dog for my diabetes and he was only 8 years old.
I guess my opinion is if you can choose and plan it do so. It makes for a much better memory for you. I had him creamated and footprint made. Hubby is working on a nive little box for them so I can still have him around.