Use the Make Super User Screen, located under SYSTEM | SECURITY on the menu.

By default, if you call the View.html page, you can view the calendar without being logged in.

If you're taken to a login screen, you can use the Setup Public Calendar Viewing screen to enable public calendar viewing.

In the SYSTEM | CONFIGURATION | SECURITY screen, delete the PUBLIC USER NAME value and click on SAVE CHANGES.

The simplest way to do this is to go to the Setup Public Calendar Viewing Screen.

See Also: Permissions for Anonymous Users (Permissions for Anonymous Users, User Preferences)