Whenever you are lucky enough to receive an invitation there are certain obligations that come with it. The first is always the obligation to reply. Always do your host or hostess the courtesy of responding promptly, as you appreciate if roles were reversed. If you accept the invitation you are obligated to attend. If you have to change your response you need to have a good reason, like an illness, a death, an injury and you need to notify the host as soon as possible. Out of respect for your host/hostess, don’t bring any uninvited guests. And lastly, you are always obligated to thank your host, extra points for a hostess gift or hand written note!
Edit to add:
** We apologize for the photo, the contents were overlooked completely**