AWWWWWWWWWWWWWWW. That is just the sweetest story!
My mom told me this weekend that the reason why Christmas was so magical to her was because she and her brother and sister would go to bed Christmas Eve and when they got up, the Christmas tree would have magically appeared, fully decorated with wrapped presents underneath. I thought that was pretty neat.
One year, my mother wrapped the presents, and for my dad was a little cylindrical package she said was sterno or something for hunting. Since it was a boring dad-present, I didn’t peak. Christmas morning came and she gave that one to me to open. I protested that it was for Dad, but she said the tag was wrong. I opened it up and there was some canned cat food. We didn’t have a cat at that moment, so I was naturally befuddled (I was probably 8). Then my father walked in carrying a siamese cat that was my Christmas present. Live prezzies are always best.
A few years later they surprised me with my afghan hound puppy. It was early - probably October, but Siddhartha was my big present for the year. I slept with him in the kitchen for a week, because he wasn’t housetrained yet. He was with me for 14 years.
StG
My parents recently told me the same thing. Apparently it was a tradition in the 1940s for the parents to set up the tree on Christmas Eve, after the children had gone to bed.
Dad’s grandparents lived in the apartment below his parents’ apartment. One year, his grandfather got drunk and bought a 15-foot Christmas tree. The ceiling in the apartments was only 8 feet high. My dad’s father got out his measuing tape and his saw, cut off the tree at the 8-foot mark, carried the top portion upstairs to his apartment, and set it up in the exact same spot in front of the window. From the outside, it looked as though the family had a single, 15-foot tree. 
Christmas Eve of 2001, everyone had gone to bed and it was just me any my then boyfriend in the living room. I had been pestering him for days to let me open one of my gifts early, (I’ve done this for years, it drives my mother batty, although she never caved in…) and he finally relented. Success, finally!
I asked to open the red package, but instead he handed me a blue package with a silver bow on it. I not-so-carefully tore off the paper, and underneath was a plain white box. By this time, I’ve shaken it, and squeezed it and I’ve run through the list of things I’d asked for trying to guess what’s in this box. I don’t have a clue.
I flipped open the lid of the box and immediately began to tear up. On the underside of the lid he has written, “Will you marry me?” and inside the white box, nestled in tissue paper is a much smaller black box.
It was just the two of us at that moment, which was perfect. And made for huge excitement the next morning…
Ah well, you will go in for that weird cold Christmas thing…
here you can ride your bike ON the pool at Christmas time… 
Whew! I thought you were going to say Grandpa was drunk enough to cut a hole through the ceiling/floor!
Kitties, puppies, engagement rings…what great stories!