Sorry, it's not easy!
Consider when 1st January is this year, and in future years:
In 2012 it's a Sunday.
It's a leap year this year, so 1st January will be two days of the week ahead next year (instead of the usual one). i.e. in 2013 it's on a Tuesday.
For the next 3 years it will move one day forward:
2014 Wednesday
2015 Thursday
2016 Friday
Since 2016 is a leap year, it will then jump 2 days again:
2017 Sunday
So a calendar for 2012 would, at first glance, appear to be valid for 2017 (as the year starts on a Sunday again). However the calendar only stays the same until the end of February, because 2012 is a leap year but 2017 isn't. This year's calendar can't be re-used until 2040!
http://en.wikipedia.o...ar_starting_on_Sunday
The easiest way to work out when your calendars will be valid again is to Google for the relevant Wikipedia pages, remembering to take into account whether your calendar is for a leap year or a common (= 'non-leap') year.
For example 2011 was a common year starting on a Saturday. Enter 'common year starting on Saturday' into Google and you'll be led to this:
http://en.wikipedia.o..._starting_on_Saturday
Looking at the 'Gregorian' list (because that's the type of calendar we use these days), you'll see that you can re-use your calendar in 2022.
Chris