According to the Emily Post Institute, as an invited wedding guest, you have the obligation to respond immediately to a wedding invitation. The term RSVP is an abbreviation for the French phrase, "Respondez s'il vous plait," which simply means, "Please respond." A couple planning their wedding needs to know how many people to accommodate, and a timely reply is the most helpful service an invitee can provide.
The wedding invitation will indicate a date along with the RSVP. This is the latest date by which guests are expected to answer. It is important to reply prior to this date because the couple has deadlines to meet for providing head counts, ordering favors and creating seating arrangements. It is impolite to assign the busy couple with the additional task of pursuing a response after the RSVP date.
Most wedding invitations contain response cards with preaddressed, postage-paid return envelopes to make the process of responding simple for guests. A typical response card will provide a place for guests to fill in their names and check off whether they will or will not attend. Some cards will have a place to fill in the number of guests who will be attending. It is poor etiquette to fill this space with more guests than were invited. The only invited guests are those to whom the invitation was addressed. If the invitation was addressed to an individual "and guest," it is acceptable to bring a date and appropriate to fill in that person's name on the response card. If the words "and guest" do not appear, inviting a date and adding a name in addition to your own on the reply card is not acceptable. The same rule applies to couples with children: If the children were not included in the address, they are not invited. It is unacceptable to call the couple and request to bring along uninvited guests.
Some wedding invitations will state, "Regrets only." This means only those who will not attend are to respond while those who plan to attend do not reply.
Along with the response card, phone calls and emails are also acceptable, but only when specified on the invitation. A couple accepting these responses will state as such and include a designated phone number or email address. If these options are not indicated on the invitation, the response card is the only appropriate way to RSVP.
When sending regrets, it is proper etiquette to send a congratulatory note and a gift. If the gift is not sent at the time of the response, it is best to send it prior to the wedding or at the very latest, within three months from the date of the marriage.