Where is this calander (Excel?). Is it in a network forlder. Is it a master? Who can access? If you would have a flow chart then we could comment.

How is the request being send in.

If there is a master one could check the latest revisions.
You get a list of changes
Once approved you send out an email automatically

This all requires VBA code. If you are not able to Maintain this yourself (or someone in the company) you should rethink this option. You could (hopefully) get away with just Worksheet formula's only, but all depends on your requirements.