For those that don't know, December 23 is an important date on the Mormon calendar. On December 23, 2005 Joseph Smith was born. By his own admission, Joseph Smith is the most important man to have walked the earth, save perhaps Jesus Christ himself. The appropriate greeting for this day when meeting a Mormon is, "Merry Smithmas to all, and to all many wives."
Thanks to RfM for informing me on the appropriate greeting.