If you use the most excellent Calendar module, then you probably know that it defaults to a "Month" view. Here's the easy way to change the default view to something different.

In your calendar's main view, click on the "defaults" tab, and then click to edit the "Date: Date (node)" argument. Simply change the "granularity" setting to whatever you want the default view to be and you're golden.

(I found this tip deep in the Drupal.org forums.)