...was yesterday, 5th January. There was an ongoing debate on the BBC Radio 4 Today programme about it this morning, but they had an authoritative ruling from an unnamed Church of England spokesman that it was yesterday. Didn't know we had such authority!
Actually it's easy. Christmas Day is the first day of Christmas (obviously), so if that's day 1, then Jan 5 is day 12. For Christians, it's a little easier as we have an alternative calendar to work from. Christmas ends when the season of Epiphany begins on Jan 6 (unless you're Orthodox, in which case Happy Christmas!)
If you forgot all this and you're decorations are still up, don't get superstitiously stressed. Many churches leave their crib sets out and add the magi/wise men/kings today to mark Epiphany, and if you are really slow off the mark, draw comfort from the story my grandad told me that in rural Lincolnshire, people used to leave their decorations up until Candlemas (that's Feb 2 in new money) You've got plenty of time!