Hello @Penelope15,
There isn't a way to put a time window on the property type, like "Private room" usually but "Entire property" in,say, December 2016 for example.
I think that the only way is to make a new listing for the entire space and have all dates for that blocked except for those when you'll be away. Then block your away dates on your private room listing.
And as you realise, you don't want someone to search for "entire property" and get the entire place for the price of a room.
You also don't want people to search for "private room" and see your place priced for the entire space before you go away.
Given that people use "entire property" and "private room" filters in their searches and don't always read the comments or descriptions you can't rely on putting that information in the description only.
Steve.