ICYMI, September 18th is a day to be celebrated. Why? We're glad you asked — it's National Cheeseburger Day! As in, yes, it's a thing and, if you haven't been cheers'ing your friends and family with cheeseburgers on this special day, you're missing out. There are many reasons to know and revel in this date aside from simply rejoicing that such a day exists. Like, for instance, the fact that some restaurants serve up free burgers on National Cheeseburger Day. Or how about the fact that this…