First of all, is Christmas really the best day of the year? I think this is only true to some people.
But for those who consider Christmas as the best day of the year, maybe it's because Christmas reminds them of their childhood (given they had good memories where they were still kids), when the family gets together and design the Christmas tree, cooking together, have a big special dinner, bonding over stories of their parents back when they were younger, putting up their socks for the toys Santa leaves for them when he pays them a visit, waking up to boxes of gifts under the tree, opening those boxes and finding the things you wished for. For them, Christmas probably is the one day they get to be a kid again, have a trip down the memory lane when times were so much easier and simpler.
I like Christmas, actually. Because everyone comes together and just be a family again. Lots of reminiscing, and laughing, cooking together (my brother grilling the meat, me and my sister making spaghetti and frying some chicken), the kids just running around and making noise, the Christmas carols, firework. All of it. Makes me wish i was a kid again.