When you are the recipient of an invitation to a birthday gathering, it is proper etiquette to respond to the invitation. Your reply to the invitation will indicate your intentions to attend or not attend the event. How you respond is important. The hostess is depending on your response to properly organize her event.
Reply quickly. By delaying your response to the invitation, it impedes the progress of the host. The host needs an adequate headcount to ensure that his event can accommodate the right number of people.
Reply graciously. Short written responses can seem harsh. Instead of just saying "No" you won't be attending, offer a short explanation. For example, "Unfortunately, I will be out of town. I'm sure the gathering will be great."
Reply accurately. If you will be bringing additional guests to the gathering with you, list how many guests you will be bringing. This helps the hostess to ensure that proper accommodations are made for each guest.
Reply according to the established protocol. If the protocol on the invitation asks you to respond in writing, be obedient. If you received the invitation via a telephone call or in person, you can respond via telephone or in person. If you received the invitation via email, you can respond via email.