The twenty-seventh of December, 1991
Well, I have my answer. Yes, I am now aware of what that mirror does.
When I was staring into the Mirror earlier, I noticed that somebody was standing behind me. It turns out Dumbledore had been lurking at the back of the room invisible for as long as I'd been there, and he seemed amused, for some reason.
I told the man that I hadn't seen him, to which his response was a cryptic "strange how short-sighted being in love can make you." I deduced from this either that Dumbledore has been watching me for a while, or that he simply wasn't troubling to disguise himself. Mind you, neither scenario would surprise me, the world does tend to disappear when I'm looking in the Mirror.
Dumbledore continued; he told me that I, like many others before me (and at this point he was practically laughing, he definitely looked as though he had just told a joke), had discovered the Mirror of Erised. Funnily enough, as he said that I noticed strange writing around the edge of the Mirror. Writing which, when reversed, read "I show not your face but your heart's desire."
I think Dumbledore realised that I had put two and two together by that point, but he continued to explain that the Mirror shows nothing more or less than the deepest, most desperate desires of our hearts. That really makes a lot of sense.
Dumbledore then offered me a warning, saying that men have gone mad in front of the Mirror before, which I can totally understand. It's probably not normal for a thirty-one year old teacher to be spending hours every night sitting in front of a mirror, reflecting (!) on how my actions led directly to the death of the only thing I've ever loved, while staring at her. Most would spend the time sleeping, or planning lessons.
Dumbledore followed this up with a final statement about the Mirror being moved tomorrow, and warning me against going looking for it. Mind you, he knows perfectly well that I'm fully aware where it's going to end up (protecting the Stone. What else could it be?) so why he's telling me not to look for it is beyond me. It seemed to amuse him, though. He walked out of the room after telling me to make the most of being able to look into the Mirror tonight, muttering something about socks and tittering to himself as he went. Goodness knows what that was all about.
Either way, I'm not going to be able to see you again now until I can persuade Dumbledore to lend me his Pensieve, which looks unlikely.
All my love,