I never would have imagined that some of the kids at my church would think that Halloween is the devil's birthday. They go to a church where the sermons taught are not topical in anyway, but verse by verse, chapter by chapter Bible studies. The reason they think this is because of screwed up beliefs that their parents have from their parents' screwed up upbringings.
Halloween is NOT the devil's birthday. Satan, also called the Devil, was a created being. He was not born, like we are. He was made by God as all other angels are made. There is no human time or day that we celebrate his creation, and thinking so is just...silly. Halloween was originally a Gaelic festival celebrating the end of summer. They believed that since that night was the end of summer and the beginning of winter, the spirit world and the physical world no longer had anything separating them from each other. They dressed up in frightening costumes so that they would look like a harmful spirit and therefore avoid harm.
This is a pagan belief, yes, but after being mixed with Catholicism, Christianity, and other various myths and legends, it's turned into nothing but a night for kids to dress up in costumes and get free candy, for teenagers to sit at home and scare themselves witless in front of horror movies, and for adults to have rather questionable Halloween parties. It's not the devil's birthday. That's just silly.