I was unaware of the Halloween and mirror combination. Old Halloween cards often featured the theme. Here's a few I found online to sort of match the prompt.
Looking for a photo with a mirror involved, I found one of my sister yet again...here she is on her wedding day, glancing away from the mirror for a moment. At this point, she had her husband all sewed up, waiting for her in the church so she knew who her lover was going to be. And the day wasn't Halloween, it was earlier in the year - September. She was happy and the wedding was perfect. At this particular moment however, she may have been experiencing cold feet; don't all brides get the last minute jitters? Here's where I insert my corny poem, a la the greeting cards.