This is a function defined within my calendar plugin. Because it is defined within another functiuon, it will only get loaded when that function is called (ie. when you add the tag).
What seems to be happening is that your theme or some other plugin you are using also has a function named month_comparison(). This means that when you add my calendar tag in, there is an attempt to load a function with the same name and thus causing the error.
To fix this you should firstly try deactivating all your plugins and then activating calendar and adding in the tag to a page (you should use the code editor not the visual one). If that doesn't work then your theme will be to blame.
If it does work you then need to begin activating your plugins one by one and checking for the error. When you activate a plugin and get the error, that plugin will be the one you want to get rid of.
Hope this helps