Complex sentence: Ordinary Time thus always includes the entire months of July, August, September and October and most or all of June and November; in some years, Ordinary Time includes a portion of May, a day or two in early December, or both.
Split into simpler sentences: Ordinary Time thus always includes the entire months of July, August, September and October and most or all of June and November. In some years, Ordinary Time includes either a portion of May, or a day or two in early December, or both.