It's a simple reminder at the start of Advent, that the "X" of "Xmas" originally had nothing to do with taking "Christ" out of "Christ-mas" but using a Greek symbol to represent "Christ." John Byron says:

So next time you see Xmas in a store don't tell them "hey, put Christ back in Christmas." Instead you might thank them for partaking in a ancient Christian tradition of referring to Christ with a Greek letter. I doubt many will know what you are talking about.