As all of this code is in the main() function, do not forget to call it.
if __name__ == "__main__": main()
Just this simple calendar can be used in any way that needs a calendar representation. By hyperlinking the dates in the HTML, one can easily create a diary functionality. Alternatively, one can check against a diary file and then reflect which dates are taken by their color. Or, if one converts this program into a CGI script, one can have it generated on the fly.
Of course, this is just an overview of the calendar module's functionality. The documentation gives a fuller view.