Calendars are very economical and multipurpose educational tools. They come in many different themes such as art, nature, vocabulary words, etc. They can be used to teach counting, months, days of the week. They can be used for creative inspiration in art, photography or writing. Calendars are a good source of pictures of all sorts, that could be adapted for many different activities. Years ago I got some calendars in a dollar store and used them to teach the kids about different species of cats and dogs. And outdated calendars could be recycled for various paper projects of course.
So three cheers for calendars!!!