enable administrators to move an event from a course level to a site-wide level

moodle/calendar/event.php (line 167) add :

   'courseid = '.$form->courseid.','.

moodle/calendar/event_edit.php after line 82 add:

    <?php if ($form->courseid != 1) { ?>
    <tr valign="top">
        <td align="right">
        <?php print_string('changetositewide', 'calendar'); ?>:
        </td>
        <td>
        <?php
            $courses[$form->courseid] = get_string('stayascourseevent', 'calendar');
            $courses[1] = get_string('makesitewide', 'calendar');

            if ($form->courseid != 1) {
                choose_from_menu ($courses, "courseid", $form->courseid, "", "", "");
            }
        ?>
        </td>
    </tr>
    <?php } else { ?>
        <input type="hidden" name="courseid" value="<?php echo $form->courseid; ?>" >
    <?php } ?>

and remark line 109:

<!--input type="hidden" name="courseid" value="<?php echo $form->courseid; ?>" /-->

add the following strings to calendar.php file in your english and local language folders:

$string['changetositewide'] = 'convert this event to site-wide?';
$string['stayascourseevent'] = 'no. keep it at course level';
$string['makesitewide'] = 'yes. make it site-wide';

enjoy :-)