These Drupal 6 calendar files are included here as part of our Drupal source code examples project. (Please see that project page, including our promise to donate back to the Drupal open source community.)
This content is from the Drupal 6 calendar project README file:
Creates calendar displays of Views results. Create a new calendar by enabling or cloning the default calendar, changing the date argument to use the correct date field(s), and setting up the year, month, day, week, and block views with the desired styles and fields. Unlike previous versions of the Calendar module, there is just a single Date argument instead of year, month, and day arguments. The argument value will be YYYY-MM-DD for a day, YYYY-MM for a month, YYYY for a year, and YYYY-W99 for a week. There is a default option to set the argument to the current date when the argument is empty. A calendar display creates calendar navigation and links to multiple displays for the year, month, day, or week views. The actual displays are created by attaching calendar views that use whatever styles are desired for those pages. Calendar views are attachments to create the year, month, day, and week displays. They can be set to use any style, either a calendar style or any other Views style, like teasers or lists. If you don't want to use one of them, don't attach it to anything. Only the attached views will show up in the calendar. A calendar block will create a calendar block for the view results. Attach a block view to the block and set up the desired style in the block view. If the Calendar iCal module is enabled, an iCal feed can be attached to the view.
These are direct links to the Drupal 6 calendar project source code files included in this project: