How can I override the Calendar module's theme functions?
You can create your own custom module and define your own copies of these preprocess functions.

For this example we'll call your module mysite.

In your mysite.module file you would define:

mysite_preprocess_calendar_mini(&amp;$variables)
mysite_preprocess_calendar_month(&amp;$variables)


And then modify the contents of the variables in these functions as needed. Copying the existing preprocess functions might be a good place to start if you're not sure what you want to modify.

Check out the preprocess documentation for some more information about how this system works.