The holidays are a time for joy and kindness, so your message inside a Christmas card should reflect those values. You could start by writing a simple greeting, like "Merry Christmas" or "Happy Holidays." Then, you can express your gratitude for the recipient's friendship or support, or share a memory from the past year. Finally, end with a note of goodwill and hope for the future, wishing them all the best in the coming year. With a heartfelt message like that, your Christmas card will surely make their day!