Long ago, as you are no doubt aware, Jesus was born in a feeding trough in a barn. This was unpleasant for him but even more unpleasant for his mother, because as bad as it is to be a newborn baby lying on scratchy straw that animals have drooled on, it is worse to travel all day on a donkey only to be told at the end of the day that there is no bed for you to sleep in, and then to have to give birth in a stinking barn. Poor Mary was very tired. But she couldn't sleep, because her little son was wailing, and that's an awful sound.
Mary tried hard to comfort her baby, but to no avail. Because he was a special baby, all the animals were crowded around to look at him and breathe on him. His mother rocked him and sang to him but he didn't sleep; he just cried and cried, and Mary cried too.
At last a little tabby cat made its way to the manger. Soundlessly, it leaped into the straw and curled itself around into a soft pillow for Jesus' head. At once he stopped his fussing and cuddled up to the kitty, who was very warm and inviting; and its gentle purring soon soothed him to sleep. And Mary went to sleep too, but not before petting the cat very carefully, and whispering her heartfelt thanks.
And that is why, to this day, all tabby cats wear the letter M on their foreheads. It is Mary's initial, and a sign of her gratitude to them forever and ever.