A hand-written note is the proper way to accept an invitation to a bridal luncheon, unless the host or hostess indicates otherwise by providing an email address on the invitation. Etiquette dictates that an invitation should be accepted or declined promptly, within a few days of receiving the invitation. As with all social writing, the acceptance letter should keep a light and gracious tone but should stay formal. If an RSVP card is provided, it is acceptable to write a quick acceptance note on the back of the card. Otherwise, use standard stationary from an office supply store.
Start your acceptance letter with the word "Dear" and the host or hostess' title and last name followed by a comma. So if the hostess is Jane Doe, your letter will start "Dear Ms. Doe," after which you will skip a line. If you are closely acquainted with the host or hostess, it is acceptable to use his or her first name with no title.
Write a sentence thanking the host or hostess for the invitation.
Write a sentence accepting the invitation. Express your happiness about attending the event in a line such as, "I would be delighted to attend the luncheon" or "It is my pleasure to attend the luncheon."
Express your expectation that the luncheon will be a joy to attend, you are looking forward to spending time with people you have not seen in awhile or you hope this will be the first of many events celebrating the upcoming wedding. An uplifting phrase is best when ending an invitation acceptance.
Finish your letter with a closing such as "Sincerely," followed by your signature.