You should notify them immediately and they will reschedule you to another day. You may have to wait a while before you are rescheduled to a new oath ceremony date.
See the following information on the CIC site:
http://www.cic.gc.ca/english/information/faq/citizenship/index.asp#ceremony
What if I am unable to attend the ceremony?
Contact the citizenship office immediately. You will be rescheduled for another ceremony as soon as possible.
If you do not come to the ceremony and do not contact the citizenship office within 60 days, your application will be considered abandoned and your file will be closed.
