Why was the white witch mad when she saw a squirrel and his party feasting in the woods?

The squirrel and his party was feasting because Father Christmas had brought them the food, and given them reason to celebrate. This meant that the White Witch's power was weakening. For decades, she had made it so that it was always Winter and never Christmas in Narnia, but the very fact of the party feasting of Father Christmas's gifts could mean only one thing - Aslan was on the move. That meant the White Witch's reign was coming to an end.