The rules surrounding reply etiquette remain the same as they have always been. Having RSVP etiquette is a perfect opportunity to show that you are considerate and respectful of other people’s time and thoughts. The term "RSVP" on an invitation stands 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 she feels like it.
Having RSVP etiquette means simply replying in the manner and timescale that your hostess has requested which then enables her to plan the event. If the hostess does not specify a deadline in which to respond, then you should ideally reply within two, and definitely a maximum of five, days.
If you need to check with your spouse/babysitter, then it is good RSVP etiquette to acknowledge and thank the hostess for the invite and let her know when you will respond fully, e.g., “Thank you for the lovely invitation, it sounds wonderful. I will check with 'John' and let you know on Friday.” Ensure that you do let them know by "Friday" at the latest.
If you are unsure as to whether you can attend, call the host and explain you may have a prior commitment but if not, you would like to be there. It is polite to ask them what their deadline is but normally three days before is acceptable–and be sure to let them know.
If you have already accepted an invite but can no longer attend then you must let your host know as soon as possible. You will need to offer an explanation and be genuine with your regrets. Simply not turning up is bad form.
If you have declined an invitation but are now able to attend then it is acceptable to call the host and ask. Note this should only really be done if the event is a large party where an extra guest isn’t going to cause too much upset to the plans.
If you must decline an invite you don't need to over explain. A simple “we are unable to accept your kind invitation due to another engagement” is sufficient.
If you forget to RSVP, then you should call the host straight away and apologize. After the phone call send a hand written note and ensure you take a nice gift with you if you are attending.
Source: Etiquette and Manners for the Contemporary Woman Photo: Joe Zlomek