Ah, ok it looks like the parent child relationship is used instead.
Execute the following in your database
select ID, ParentID from SiteTree_Live where ClassName = 'CalendarEvent';
I am seeing the likes of this:
+------+----------+
| ID | ParentID |
+------+----------+
| 6634 | 6418 |
| 6600 | 6418 |
+------+----------+
2 rows in set (0.00 sec)
If you have multiple ParentIDs then you may well have more than one calendar.
You can check the parent type like this
select ID, ClassName from SiteTree_Live where ID=6418;
+------+-----------+
| ID | ClassName |
+------+-----------+
| 6418 | Calendar |
+------+-----------+
1 row in set (0.00 sec)
Change the ID obviously depending on your results