Sometimes it's unavoidable, but there are ways to redeem yourself.
Receiving an invitation to a wedding is an honor, but it’s also filled with etiquette hurdles that may be unfamiliar to you. Perhaps the most important responsibility for a potential wedding guest is the RSVP. It’s best to send in your response card (or submit it online) as soon as you know whether or not you are available, but it’s especially important to let the couple know by the deadline they set. The reason a date is given for an RSVP is because the caterer, venue, and other vendors, such as the rental company, need to have a final headcount for the wedding. In order to make sure everyone in attendance has a seat and a meal, accurate RSVP counts are necessary.
However, as much as we wish it weren’t the case, things can happen that make a guest need to cancel when they already said they were attending. With any luck, the change in plans is early enough that it does not affect things like the seating chart or meal count. Unfortunately, sometimes an emergency strikes only days before the wedding. In those instances, you should let the bride or groom that you are closest to know as soon as possible. It’s better for them to know the day before than to realize at their reception that a loved one was a no show. Make sure you are the one who gives the message, rather than just having another friend or family member convey your regrets.
There is still protocol to follow after the wedding as well. If for some reason you did not do the above, it is absolutely imperative to contact the couple after the wedding. Otherwise you risk them thinking you completely forgot about their big day or simply did not care. In either case, it is a kind gesture to reach out after the nuptials as well, perhaps offering to get together (if geographically convenient) so you can hear about the celebration.
While some people send a gift to every wedding they are invited to, others only do when they attend – which is fine. Now, giving a present is never required, but if you would have given a gift if you attended the event, you should still do so after backing out at the last minute. After all, the couple already spent money on your food and rentals, and it is not their fault you were unable to attend. If the reason you could not attend was a major emergency, such as a death in the family or a very serious illness, it’s completely understandable to take some time for yourself before reaching out. However, if your excuse for not attending at the last-minute was due to something like a work problem, extreme weather, childcare, or a minor illness, then the sooner the better.