Sure, little girls look adorable in their Christmas dresses. They make sweet pictures. Maybe you wear the dress to church on Sunday. Maybe to Grandma's house. After it's worn 2 or 3 times, you're done with it. It's an expensive investment if you pay full price.
Yard sales are a great way to stock up on things like that, but if you don't get an opportunity to yard sale often, or you didn't find an adorable Christmas dress in your little girl's size, thrift stores can give you good deals. It's cheaper, it's better for the environment, and it supports local rather than big box businesses. Win all around!
My daughter owns an online thrift store that is perfect for helping you find great deals on clothing--both children's and adults.
As of this writing, there are some super-cute Christmas things available. (Of course as things are bought and new things are added, things may be different, depending on when you're reading this.