It is generally a big deal to miss responding by the RSVP date on an invitation; especially if you plan to attend. Invitations that include an RSVP date indicate that the host needs to know how many will attend. Please remember that RVSP, (répondez s’il vous plaît) means please respond to the invitation, either yes or no. The event could be a wedding and a definite number needs to be turned into the venue. Or, the event could be a dinner party and the host needs to be able to accurately calculate the amount of food and drink, etcetera that they will need.
It is rude not to respond to an RSVP, and even ruder to respond late. You are putting the host in a bind and creating a potentially awkward situation if he or she has to tell you that they have already turned in their numbers, so you cannot attend. Most hosts will try to accommodate late responders, but it is not always possible and you have really imposed upon the host.
You should be appreciative of the invitation and be respectful to the host by responding on time.
However, there are times when the event is very casual and the number of guests might not matter as much, for example, a backyard BBQ with potluck. Maybe your plans changed and you can make the event after all, and you know it’s a very casual get together—then it might be ok to contact the host and see if you can attend.
When you receive an invitation, mark the RSVP date on your calendar and set a reminder to contact the host. We are all very busy, but this will take only a moment and it will save you from being disrespectful and rude to someone who was kind enough to invite you.