The calendar business has grown into a $9 billion industry since Pope Gregory XIII devised the modern calendar in 1582 and has spawned a trade group that tracks such statistics.

The 2-year-old Calendar Marketing Association, based in Libertyville, Ill., says there are more than 3,500 calendar titles available for 1991 on subjects ranging from Arctic animals to Ziggy. Those are in addition to the millions of free calendars distributed annually by advertisers.The average household has about a half-dozen calendars, a third of them received as gifts. Animals and outdoor themes are the most popular subjects, according to the association, but the best seller for the past four years has been Gary Larson's "Off the Wall" desk calendar. It sold more than 2.2 million copies in 1989 alone. The CFA says a retail success is 20,000 sales.