Perhaps the biggest misconception about “Respondez s’il vous plait” is whether or not you need to respond to the invitation if you are not planning on attending the event. You must reply, even if you are declining the invitation.
Always reply as soon as you can, before the date you were asked to reply, if given, or well before the event. Always let your host know that you will be bringing a guest if you were invited to do so. And, depending on the event and your relationship with the host, ask if you can bring or do anything to make the host’s job easier.
Responding and then bailing 48 hours before the event because of non-life threatening or lame reasons will blacklist you in the books of party-lovers everywhere.
If you reply and say you can come, honour that commitment, but if something comes up and you won’t be able to attend, tell the host with as much notice as possible so that they can invite someone else.