How to Make a Christmas Riddle Card

For years I've been making up clue games for my siblings and relatives (that usually result in headaches for them as they try for hours to solve the riddles and get to the next clue), so I thought it'd be fun to extend a bit of that cipher-making into some Christmas cards. It's just kind of a fun way to make your family and friends think for a moment when they open their Christmas cards. However, the riddle on this card is pretty straight forward and easy enough that there shouldn't be any headaches involved for the recipients. Though if you do want to make it a bit more difficult for them, you can leave out the underscore completely so it takes just a bit longer to figure out what letter is missing.