I could begin by giving you the French words behind the acronym, RSVP, but that’s really unnecessary to get the point of the request when you see it on the bottom of an invitation. When a host writes these letters, which are usually accompanied by a date, they are asking you to let them know whether or not you plan to attend the party on the invitation. This doesn’t mean to tell them if you think of it; only if you feel like letting them know; or at the last minute when you see if a better invitation arrived on your doorstep. No, it means to let the host know as soon as possible, and by no later than the date stated on the invitation whether or not they can count on your showing up for their event.
But why, you might ask, is this such a big deal? Perhaps you’re thinking that you don’t eat very much so it won’t require a lot of extra food for you to drop by at the last minute. Or, maybe you’ve decided that RSVP is too formal for our more relaxed culture. Nobody worries about these things any longer. Well, maybe if you’re a teenager or college student who is accustomed to last minute party text blasts, you could be correct. But when you enter the adult world it still matters, and it’s not just for the parent/grandparent generation.
Put yourself into a host’s position. They need to buy food, beverages, party goods, and possibly party favors. They need to set up the right amount of chairs so that guests aren’t spending their time playing musical chairs during the party as they look for a seat so they can stop the juggle of food plate and beverage holding with eating and drinking. If the party is a formal event held in a catering hall, firm numbers need to be given to the caterer by a certain date and the host is expected to pay for you even if you don’t show up. Is that fair?
The only exception to the rule of replying to your host with either a yes or no response is if the invitation states “regrets only”. In that case your host is planning a more relaxed event, assumes most people will attend, and only cares to know if you won’t be there. I never recommend that hosts put that on their invitations because that invites an even more lax response rate from guests. However, your responsibility as a guest is only to tell your host if you cannot attend the party.