Found these tips on a blogging site, but couldn’t find them again to give the original author the credit. But to unhide the event content type from the OOTB SharePoint 2007 Calendar, follow these steps:
1.Go into the ‘List settings’ for an existing calendar on your site
2.Click on ‘Advanced Settings’ and tick yes for “Allow Management of Content Types”
3.This then shows the ‘Event’ content type in that calendar’s List Settings.
4.Click on the word ‘Event’ in the list of content types.
5.At the top of the next page it tells you that the parent is also called ‘Event’ – click on its link.
6.The following page tells you that the parent content type is in Group “_Hidden”.
7.Click on the “Name, Description & Group” link and then it allows you to put the ‘Event’ content type into a new group.
I put it into “List Content Types”. If I then go to create a new content type, the Event content type is selectable (under the parent of List Content Types).