Old calendars never really go out of date.

Filed under the category "I didn't know that" there are only 14 combinations, including leap years, of months and days on the Gregorian calendar. That opens up a market for the recycling of old calendars. There is even a website - time and date.com - that will calculate the matching years for you. For 2009 there are 12 repeats going back 118 years!

From wisebread.com