1) The custom is to decorate the house and synagogue with greenery, as a commemoration of the event of the giving of the Torah at Mount Sinai where there was greenery on the mountain. One also puts blossoms and flowers as a remembrance of the giving of the Torah when the whole world was filled with the scent of spices. (Shulchan Aruch, Mishne Berurah and Kaf Hachaim 494)
2) If one did not decorate his house before the festival, one may even decorate it on the festival itself, so long as everything was picked before the festival, and he intended beforehand to use them for this purpose (as if he did not have this intention, they are Muktzeh and may not be moved). (Kaf Hachaim ibid)
