editting the standard calendar

Hi all, after extensive plugin research trying to find the perfect calendar I decided I just want to alter the basic one instead to suit my needs.

Maybe its a stupid idea but Im trying to learn php as well so no better opportunity.

The problem is Im having trouble locating all the relevent files that are associated with generating the calendar.

All the google searches are bringing up other plugins and events calenders.

Basically Im looking to give different colours to the background of each date based on category.
Im also implementing the ability to make future dates appear.

Any help would be great on the location of these files thanks.