Idiom: Rain on Someone's Parade
John: I'm so excited to see the new Iron Man movie tonight! Let's go to In-n-Out before showtime and get burgers to have dinner in the theater. That sounds like a great idea, right?
Bill: I hate to rain on your parade, but they won't allow outside food in the theater. We can go their early and eat first.
Mom: Aren't you excited?! Our camping trip is only 2 days away! Just imagine all the fresh air, hiking, and the sounds of nature! I can't wait!
Teenager: No. I am not excited. There's no WIFI in the woods. I won't be able to text, watch videos, or send Snaps. That doesn't sounds fun at all.
Mom: Well I won't let you rain on my parade! It's going to be great! You'll have fun. I promise!
Meaning: to "rain on someone's parade" is to ruin their good mood or fun plans.