This is a secret message from Santa, explaining that two of his elves will be visiting to find out if the reader is being good.

GET AS PDF
For your convenience, the mirror writing part of the letter reads as follows:

Well done for finding a mirror, my dear! Now you can read my magic mirror writing, I shall tell you a special secret. Two of my elves are coming to keep an eye on you this week, so that they can write me a report about how good you’re being. I’m telling you in advance because I really want to bring you some presents on Christmas Eve, and now that you know my elves are watching, you’ll be able to make sure you’re extra well behaved, won’t you? That way, they’ll put you straight on the Nice list! I’m afraid I have to go now, as I don’t want anyone to see me writing this secret letter. It’s for your eyes only! Do heed what I’ve said and keep an eye out for my elves – even though they’re invisible you can sometimes glimpse them, like a flicker in the corner of you eye. Good luck! Lots of love, Santa Claus x