1. Keep people food out of the reach of your pet and ask your guests to do the same;
2. Make sure your pet doesn't have any access to treats or candy, especially those containing
chocolate, grapes/raisins, onions or other toxic foods;
3. Don't leave your pet alone in the room with lit candles, a decorated tree or potpourri;
4. Keep holiday plants (especially holly, mistletoe and lillies) out of reach of pets;
5. Consider leaving the tinsel off your tree if you have a cat;
6. Secure your Christmas tree to keep it from falling over if your dog bumps it or your cat climbs it; hanging lemon-scented car air fresheners in the tree may deter your cat from climbing it;
7. If your pet is excitable or scared when you have company, consider putting your pet in another room with some toys and a comfortable bed, or provide a safe place for your pet to escape the excitement (such as a kennel, crate, scratching post shelf)
source: AVMA - American Veterinary Medical Association