Any time during the Christmas season, from late November to early January, can be great for watching it. You can make it a family event on a cozy winter weekend afternoon, with some hot chocolate and cookies. It's a story that really captures the essence of Christmas, so whenever you want to feel that festive cheer, it's a good choice.
Personally, I find that watching it on Christmas Day, perhaps after opening presents and having a big meal, is really enjoyable. It helps to continue the celebration and gives you a chance to relax and enjoy a heartwarming story. Also, if you have guests over, it can be a nice shared experience to watch it together.