When you RSVP to an event that lets the Event Host know you are coming and to plan for you according, food, drink etc. You should again confirm your RSVP a few days before the event.
If you have RSVP'd to an event and not attending it doesn't really matter why you can't attend the fact is you won't so just let the host know to cancel your RSVP, you don't really need any reasons to give even if you have changed your mind about going. To just not show up does not sit well with the host or especially with me. So if you can't make it do the right thing and let them know! But try your best to go... you will have a good time and be happy you went.