"Camille, I'm going to get the mail!"

The shrill voice of my mother drifted upstairs to my bedroom, where I sat at my desk, working on homework. I winced, glancing at the calendar laying open on the floor. It was Friday the thirteenth.

Today was the day. It was time for my mother to be replaced.

I remember the first time it happened. It was May 13, 2011. Mom called up to me like she always did, telling me that she was going to get the mail. But when she returned, there was something different about her. Her normally bright, brown eyes were dull and glazed over. For the following week, she acted very odd. The way she smelled, the way she did her makeup, the way she spoke, it just wasn't like my mom. She was just... different.

A week later, on Friday the twentieth, my "mom" went to get the mail again. But when she returned, she was back to her usual self. Her cheerful, bright, vibrant self. It was as if nothing had happened.

At first, I thought I was crazy. I thought I had imagined the whole incident. But three months later, it happened on the next Friday the thirteenth. For nine years, this cycle has repeated. She's replaced for a week, then she reappears. 

No one else seemed to notice. Dad has never made any comments. Mom's friends never seemed confused when they came over for luncheons. So I never brought up the matter. I mean, how would the conversation go:

"Hey Dad! For nine years, you've slept next to a woman who isn't your wife for a week following every Friday the thirteenth!"

They'd think I was crazy.

Was I crazy? Perhaps. But... I didn't think so. My gut told me something wasn't right.

Shaking myself from my thoughts, I rose from my chair and headed for the window in the loft overlooking the pitch-black street. Many years ago, I started watching her on Friday the thirteenth to figure out if a swap had taken place. The problem was that the view of the mailbox was blocked by a large pine tree. I'd tried different windows in the house, but the angles didn't lend themselves to seeing properly.

Besides, it was late at night.

So I watched my Mom from the upstairs window as she strolled to the street, dressed in her black trench coat and hat. For a split second, her figure disappeared behind the pine tree, and I let out a sigh of defeat. I supposed I'd never know the mystery of my disappearing mother.

I was just about to leave the window when something caught my eye. I leaned in closer, squinting. In the pale moonlight, I could see my mother's imposter. Her eyes were glazed over, brown pits devoid of emotion. My eyes drifted to the gleaming object in her hand.

I gasped. Clutched between her fingers was a shiny, silver knife.

And blood was dripping off of it.