For you to understand what black plate changes are, it is better to first understand the litho print process.

Litho printing is where the print document you want to produce is placed on a plate which is then covered in ink and used for printing. This process can be used to print on paper and other materials. Most litho machines use four plates (CMYK) to make up a complete image. The material receiving the image or words you want to print is placed against the plate and the inks are absorbed accordingly on the material.
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​For a full-colour plate change calendar, you will require 12 (Months) x 4 (CMYK): That's 48 plates that will make your production cost extremely high.
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Black plate change is the modification of a print document that requires only one plate change. Basically, anything that will change from page to page must be set up in 100% black ink (K). A black-plate change is inexpensive compared to changing a four-color document.
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For a black plate change calendar, you will require 12 (Months {K}) + 4 (CMYK): That's 16 plates that will make your production cost substantially lower.
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If your artwork has been reported for black plate changes not met, here is how you can fix it.
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​TIP: If you scroll through your pages, the only elements that may shift or change must be 100% Black (K) value.