Not The 13th

Had a friend warn me to be careful because today was Friday the 13th. I told her it was Friday the 12th. She tried to play it off by saying Europeans write the date differently than Americans. So I wished her a "Merry Christmas" since to her it is December 4th (European date: 12-4) and to me it's April 12th (American date: 4-12.) Still not sure where she got the 13 from though.