I can’t take my Christmas decor off my mansion. Even when I hold down to switch it doesn’t take..

Normally when you hold down to switch you can choose between the event decoration (xmas, Halloween, etc) or "no decoration". If the "no decoration" option doesn't show leaving you incapable of taking the event decoration off, it means that further in the game something will happen to create a normal decoration there. E.g.: the fence will only be replaceable after Bosworth crashes the car into the old fence. The 2 pillars by the main entrance door will only be replaceable when you're decorating the office area and Jia projects big topiary plant vases to bee put there. You can't remove the event decoration in the meantime. It's annoying, I know.

I had to take each one off one at a time. I could not remove them all as a group. I hope that helps.

You have to manually remove them, from the selection list where you choose your furniture.