It should be fairly obvious (it wasn't to me) that there are only 14 permutations of calendar.
Therefore it is very handy, for retronauts such as ourselves, to know which year of the past had the same calendar as the present. Then we can re-use, or buy, those excellent old calendars from our favourite eras without having to deface them with scribbled modifications!
The following Instructable will show you how to use Excel to create a list of compatible years, or you could just cut and paste the image to save yourselves the effort. I tried it in OpenOffice and it doesn't work so that'll teach me to be a cheapskate.
Use Vintage Calendars
Instructable and image by Instructables member,
KEUrban