How do the Christmas decorations work?
If this is not specified on the item, the duration is infinite.
To use them you simply have to put them on your shelf in the fief, and you can change them any time you want (take care not to destroy your ancient decoration)
The only concern I have about garrison decorations are when the next extension will be available, if there is also a garrison in it or something alike, I don't know if decorations will be synchronized.