The term "RSVP" on an invitation is an acronym, or more accurately, an initialism, for a French phrase, repondez s'il vous plait, meaning "respond if you please." It is a polite way of requesting a response to the invitation, not a suggestion to the invited guest to respond if he or she feels like it. Some invitations, like those for weddings, include a card and an envelope to make responding even easier. The phrase "response requested," which means essentially the same thing, may also be used.
If a party is being held locally, the invitation may be the first that a guest hears of it. Those who live in a distant location and whose presence would be genuinely appreciated should be made aware of the date and location of the party before receiving the invitation, giving them plenty of time to make traveling plans. This is sometimes indicated by a "save the date" card, which is sent out before a formal party invitation to make guests aware of the upcoming event. Invitations to important events, like weddings, graduations, and initiations, are also sent out to distant family members as a courtesy to alert them to the event, but these guests are not always expected to attend.
When someone receives an invitation to an event that he or she does not plan to attend, and when the invitation includes a RSVP card, he or she should check "not attending" and include a brief note expressing his or her regrets. If no response card is included, the invitee should call the phone number included on the invitation or mail a brief note in response, expressing his or her regrets. People do not need to delve into their reasons for not being able to attend, but they should respond within three days of receiving the invitation so that the hosts know not to expect them.
If the invitation is to an event which the person plans to attend, he or she should fill out the card to indicate this, and include information about who will be accompanying him or her. Guests should do this promptly, ideally within three days.
People should take care to read the invitation carefully, as it will provide information about who all is being invited. "Ms. Jones and friend" or "Ms. Jones and guest" suggests that Ms. Jones and another individual are invited. "Mr. Smith and family" suggests that Mr. Smith, his partner, and his children are invited, while "Mr. and Mrs. Smith" suggests that the children are not invited. In some cases, only "Ms. Jones" will be invited; when an invitation does not mention a guest or ask how many are attending, it's best for the invitee to assume that additional, unnamed guests are not invited.
Filling out an RSVP card is a basic and important courtesy. The party hosts collect cards so that they know how many guests to expect, which will help them to make seating and catering arrangements. In addition, if hosts are inviting guests to stay at a hotel or resort during the event, sending a response ensures that enough rooms will be set aside. It can also be used to convey important information, such as "my child is allergic to nuts" so that the hosts can be warned ahead of time of any unusual circumstances.