Christmas had always been special for Hermione, well; it had been in her childhood at least, when all of her family were there, her mum, her dad. Christmas had been spent surrounded by all her family, aunts, uncles, cousins, grandparents – they were all there.
As she grew up, and started to spend Christmas at the Burrow, it had become even more special for her. She was surrounded by the family she had chosen to have, and yet could still go back to her real family at any time she wanted.
As Voldemort gathered power, and she first had to spend her Christmases living in fear in Grimmauld Place, and then had to spend Christmas in a tent, Christmas had slowly lost its appeal. It reignited bad memories, ones she's rather forget.
Because what's the point of celebrating something wizards don't believe in?