Every time I had seen these on letters from Denmark, I have thought they were postage stamps. Only this year I have realised that they have not any face value. Of course, because they are not postage stamps, but Christmas seals!
The Christmas seals are specially designed stamps launched each year and sold as extra 'seals' in addition to Christmas stamps. They appeared in Denmark in 1904, as a way or raising money to help children sick with tuberculosis. Then the idea spread out around the world.
You can read more about Christmas Seals in this entry of the Wikipedia.
(This is a scheduled post. I will try to add this post to Sunday Stamps-II, even if it does not fit exactly in Religious Christmas. But probably I will be too busy with family meetings!)