R.S.V.P. stands for the French phrase, répondez, s'il vous plaît. It is translated, "respond if you please," but really means, please let me know if you're coming. It is polite to respond to invitations so that the host knows how many people to cater for. This is especially important when you're invited to formal events such as weddings where the host is paying per head and numbers are limited. However, it is just plain good manners to respond to any invitation, whether it is a sleepover, birthday party or family event. It's a good rule of thumb to respond sooner, rather than waiting until the R.S.V.P. deadline. It is also important to let the host know if your circumstances change and you can't make it, for example, if you are unwell.