Avoid the crowds by waiting for the second "Once Upon a Christmastime" parade at 10:30pm. Fireworks are at 9:30pm. Snow is forecast to fall intermittently throughout the party on Main Street USA. Enjoy the free holiday treats found a specific locations throughout the park.
Don't forget that with a "Mickey's Very Merry Christmas Party" ticket, you can arrive anytime after 4pm. By doing so, allows more time in the park and avoids the crowds that come right at 7pm.
Finally, "Cinderella's Holiday Wish" (the castle lighting ceremony) is at 6:15 during regular park hours, so if you don't want to miss that, make sure you arrive early.