I save my wall calendars.
You see, each year eventually comes around again in exactly the same date to day of the week arrangement. It is just a matter of time.
To see what I mean take a look at this table at the Virtual Perpetual Calendar site. Each year of the calendar has a number next to it that can be used to located other years that have the exact same calendar. For example, 2007 will repeat in 2018, meaning the wall calendars you are using this year will be reusable again in 11 years.
In this manner I've been able to use my 1989 calendars twice since then, which is nice since one of those is a much loved Calvin & Hobbes calendar. However, some periods before reuse are much longer. Next year's calendar last occurred in 1980, and won't be reusable again until 2036. That's such a long way out that they might actually have light rail running to Redmond by then!
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