LumpySpacedPrincess Ooooooh, nice tree, nice deccies!! 
annodomini Yes, I posted that my husband was puzzled about this when we lived in the US.
I don't know if it started there, but being from New England, people put wreaths on the door and leave them up till way after New Year, some even till March!
In colonial days I read once that they are a sign of hospitality and because of difficulty getting round in the snow by horse or on foot, it took a while for visitors to make their holiday visits. I have never been able to verify that.
Where we lived some left their outdoor lights etc up until spring, but did not turn them on after the 6th Jan. It could be really cold and the wires would get brittle, or snow too deep to get up on the ladder, so they'd stay on the house till it was milder and sometimes that was spring.
In Williamsburg Virginia, they have a wreath walk as the houses are not open. These are gorgeous!
www.sayhellotoamerica.com/colonial-williamsburg-christmas-wreaths/