I am amazed once again how many people don't know what RSVP means or delibertly decide to ignore it. If you receive an invitation - to a dinner party, wedding, gala, baby shower, etc you must let the host know if you plan to attend. All invitations will have a date and to whom to reply to.
According to Wikipedia: Répondez s'il vous plaît or RSVP is a French phrase that translates to "reply, please." It is with this meaning that invitation cards and similar documents are often marked with "R.S.V.P." It is standard practice to reply to an RSVP request whether confirming attendance or declining.
According to Emily Post: From the French, it means “Répondez, s’il vous plaît,” or, “Please reply.” This little code has been around for a long time and it’s definitely telling you that your hosts want to know if you are attending. Reply promptly, within a day or two of receiving an invitation.
Most invites arrive between 4-8 weeks before the event - check your schedules, mark your calendar and let the host know as soon as possible that you are or are not able to make it.
Not only is it the proper thing to do; it the nice thing. Now Party On!