1991.
"That was a close one, Georgie," Fred Weasley grinned at his twin brother, "I thought for sure old Filch'd hear the door creaking."
"Maybe he's deaf," George grinned back, rubbing his pyjama-clad arms to keep them warm in the chilly room, "It'd explain why he relies on that bloody cat to do all his dirty work."
Fred chortled before turning and looking about the room they had just snuck into. It was one of the many unused classrooms scattered about Hogwarts. Old furniture was stacked in the corners and the chalkboard was covered with a dusty cloth. It was this that drew his attention.
"Hey George, let's write stuff on the blackboard. Who knows when they'll decide to use this room next? It could be years. It'd be like writing to the people of the future." And without waiting for his brother's reply he raced across the room to search for a piece of chalk. George found himself looking at the old furniture instead. Most of it was rather commonplace for an old classroom: broken chairs, vandalised desks and the like. But there was one thing that was out of place.
A tall, ornate mirror stood to one side of the room, reflecting the moonlight in an odd way. George could just make out the ornate writing running along its edge, but the words were unintelligible. He took a step closer to get a better look and gasped. Where he had expected to see only one copy of himself in the glass, he saw two. As a twin he was no stranger to seeing himself, even without a mirror. He reflexively looked over his shoulder to where the other red-haired boy seemed to be standing, assuming Fred had simply come over from the blackboard without his notice. But there was nobody there, and he could hear his brother giggling on the other side of the room.
He looked back at the mirror. There he was, his freckled face staring back at him, with another equally freckly face beside that. He knew it was Fred. Their similarities were great, but they had spent their whole lives noticing the subtle differences; the slope of Fred's eyebrows, the slight cowlick in George's hair. The question was, what was his reflection doing there when he wasn't?
"Fred," George called uncertainly.
"Yeah?" Fred answered, brushing chalk dust on the front of his pyjamas. The Fred in the mirror smiled, but otherwise did not move from George's side. It was only until the real Fred wandered over that the mirror reflected the truth. Fred's eyebrow rose at his brother's preoccupation with the ornate but otherwise unexceptional mirror.
"It's just us, Georgie," Fred said, waving to emphasise his point. Mirror Fred waved back.
"But I saw…" George shook his head as if to clear it. Mirrors were always a tricky thing for twins. Perhaps he had been mistaken after all.
"Erised stra ehru oyt ube cafru oyt on wohsi," Fred read slowly, reaching up to brush each curious word as he said it. "What's that, Latin?"
"I don't think so," George murmured, frowning. The mirror was starting to give him the creeps.
"Whatever. Come see what I wrote on the blackboard!" Fred ran off, and George hastily stepped out of the line of the reflection. He didn't like looking at it by himself. He much preferred having the real Fred than the one trapped on the other side of the glass.