I do not wish to attend the ceremony. Do I have to give notice of this?

The Events office needs to know that you will not attend to be able to keep track of number of attendees and to get a view of that all invites have reach people invited. It is very important that, within the time stated (see your invitation), you give notice of your intentions to attend, not to attend or to postpone your attendance. You are allowed to postpone up to three years from the year a invitation was sent. 
You need to contact Events for the information regarding a new invitation.