Making dialog remember what you last set the all day event flag to using persist.

calendar/resources/content/eventDialog.js

@@ -193,8 +193,9 @@ function loadCalendarEventDialog()
          setFieldValue( "status-field", "ICAL_STATUS_CANCELLED" );
       break;
    }
-   
-   setFieldValue( "all-day-event-checkbox", gEvent.allDay, "checked" );
+   if( "edit" == args.mode )
+      setFieldValue( "all-day-event-checkbox", gEvent.allDay, "checked" );
+
    setFieldValue( "private-checkbox", gEvent.privateEvent, "checked" );
    
    if( gEvent.alarm === false && gEvent.alarmLength == 0 )

calendar/resources/content/eventDialog.xul

@@ -190,7 +190,7 @@
                      <!-- All Day -->    
                      <row align="center">
                         <spacer />
-                        <checkbox id="all-day-event-checkbox" label="&newevent.alldayevent.label;" checked="false" oncommand="commandAllDay()"/>
+                        <checkbox id="all-day-event-checkbox" label="&newevent.alldayevent.label;" oncommand="commandAllDay()" persist="checked"/>
                      </row>