How to Be the Perfect Party Guest

"Always, always, RSVP. “RSVP” stands for “Répondez s’il vous plait,” French for “please respond.” When you receive an invitation that asks you to RSVP, the host or hostess is kindly requesting that you let them know whether or not you will be attending their function. Today the RVSP has come to be seen as optional. Some men believe you only need to call if you are coming; some think you only need to call if you are not coming; and some do not think you have to call either way. Sometimes RSVPing is avoided because you do not want to face the awkwardness of telling someone you are not coming and making up an excuse about why."

http://artofmanliness.com/2008/12/17/how-to-be-the-perfect-party-guest/