Show the calendar.
It loads the specified month's events, holidays, and birthdays.
It requires the calendar_view permission.
It depends on the cal_enabled setting, and many of the other cal_ settings.
It uses the calendar_start_day theme option. (Monday/Sunday)
It uses the main sub template in the Calendar template.
It goes to the month and year passed in 'month' and 'year' by get or post.
It is accessed through ?action=calendar.