Lillian Eichler,
Book of Etiquette, Volume 1, Part 2…
(1921)
“ No matter how informal, an invitation should always be acknowledged within a week of its receipt. It should be a definite acknowledgment—either an acceptance or refusal—and no doubt should be left as to whether the writer intends to be present or not. An invitation must always be answered in kind; that is, a formal invitation requires a formal reply, following closely the wording of the invitation. The informal invitation should be cordial enough to warrant a cordial and friendly reply; both invitation and acknowledgment should be free of all stilted phrasing. ”
