Mark your calendar spelled correctly if there's an A at the end of it, incorrectly if there's an E. There are many types of calendars in the world—such as the Persian solar, the Islamic lunar, and the Hebrew lunisolar—and many occurrences of the typo Calender*. In fact, we found so many (1,243 in OhioLINK) that it's pretty likely some of them are either proper names or foreign/antiquated spellings. Limiting the search to Calender* + Calendar* reduced the count to 93, which is still quite a few, but then calendar is notable for being a commonly misspelled word. There are over 200 examples in my own library's catalog. So don't flip out. Hang in there. Tomorrow is another day. (Photograph of Eugene Callender by Annelise Rostig, from the website Maximum Security: The True Meaning of Freedom by Alan Gompers.)
P.S. An astute reader informs us that a calender is a machine in which paper or cloth is made smooth and glossy by being pressed through rollers; and to calender is to press in the rollers of such a machine.