Mary is my favorite dog to read about. If I can have half the relationship and do right by my dog as you always have for Mary, I might live on knowing I really am a good human being.
Mary is the epitome of dogs to me, and the relationship you both share I am sure could not have possibly been greater. And now that you're positive it is her time, you can take solace in the fact that she really did have a dog's perfect life.. there will be closure, peace, and confidence, and you can go through this knowing for certain, without a doubt, you did right by her. The best.
The relationship you hold with Mary is a totem and an inspiration to anyone who is so lucky to have had and sincerely known a dog. Maybe the tears will never stop when you think of her, but you can always hold it in your heart that Mary was never wronged. That love was all she ever knew, and any living creature should be so goddamn lucky.
..Perhaps you already do, but please understand there probably can't be something more perfect. You even mentioned the weather(summer?/fall) being good to Mary this year. And after Monday, Mary can go on without ever remembering all those strings she had attached, and you can rest easy having known her by your side.
You will both be in our thoughts for some time.