A formal reply to an invitation includes three points: your name, whether or not you plan to attend, and the event details. A formal reply should be written in third person and delivered as soon as possible, or at least two weeks prior to the event, according to Emily Post.
Printed RSVP cards are included in many formal event invitations. If you receive one, simply fill in the card, and mail it back to the sender. If no RSVP card is included, you can write your own matching the tone and level of formality from the invitation. If you know the host well, you may also include a short personal note expressing your appreciation for being invited or a congratulatory statement, according to MyGoodGreetings.com.
If you cannot attend, do not feel obligated to explain why you are unavailable. If you do plan to attend, ensure you include the names and number of guests who are attending with you when you send your reply. If you find that you must cancel, notify the host as soon as possible so she can adjust her plans accordingly. Most importantly, if you tell somebody you are attending an event, mark your calendar, and ensure you arrive on time.