Acceptance of a social invitation is not a legal obligation;
your RSVP just communicates your best intentions to attend.
More and more often I have heard that hosts/hostesses are not receiving firm indications of whether guests plan to attend their parties, even when R.S.V.P. is boldly printed on their invitations. This leaves us with a couple of choices. First it could mean that rudeness is a increasingly growing trend in our society or that people no longer understand what the term R.S.V.P. actually means. I prefer to think positively and assume that ignorance of the term is the case versus rudeness. I know it's hard to believe ignorance to be good, but better that than rudeness!
R.S.V.P is French for "répondez s'il vous plaît" and is an acronym that loosely translates to Please Reply (one way or the other)! Check your calendar and ask yourself how you truly feel about attending this event. Don't keep your host in suspense, reply as soon as you are sure one way or another. To be proper, you should reply in writing, by hand. But you may reply in the same format that one was invited; for example, an email invitation can be answered by email, etc... R.S.V.P.'s can sometimes be oral or written, but are always determined and based on the level of formality of the invitation. You wouldn't just pick up the phone to reply to an invitation from the White House or the Pope now would you? These situations and many others require a written response.
Filling out an RSVP card is a basic and important courtesy. The hostess collect R.S.V.P. cards so that they know how many guests to expect, which will help them to make seating and catering arrangements. Your hostess needs to know how many are coming in order to prepare for the event properly. R.S.V.P.'s are the most important determining factor to help her do that well.
You never need to give a reason for not attending. Just let the host know whether you're currently planning to be there or not.
Communicate last minute changes immediately to your host. Maybe you become ill or there was an emergency. As soon as you know, you must get in touch with the host (by telephone is fine) to let them know you can't come, and apologize. Do you remember to communicate your intentions clearly to your hostess?