When do you take your Christmas tree down (along with your train display)?

Never before Epiphany.

For those wanting a rationalization for leaving it up longer, consider this:  Christmas starts on December 25.  The twelfth day of Christmas is January 5.  Epiphany ("Little Christmas") is January 6.  Then Julian-calendar (Orthodox) Christmas is January 7; and the twelfth day of that Christmas is January 18.  Finally, Theophany (Eastern Epiphany) is January 19!