Joyousness is the noun: the quality or the atmosphere of being joyous — that very-happy feeling that can fill a person or a whole occasion. A room can have a joyousness about it; a welcome can be full of joyousness. Like joyfulness, it is not the everyday word. People say joy or happiness when they are just talking. Joyousness and joyfulness overlap a great deal. If there is a hair of difference, joyousness sometimes keeps the “celebration / occasion” colour of joyous, but you should not treat them as two different things. You will meet it mainly in careful writing.