A dinner-party is the most formal and most important of all social functions. We may invite all our acquaintances to a ball or a reception. We may select more carefully for our teas and luncheons, but the dinner is reserved as the greatest compliment to be paid those we wish to honor. Therefore an immediate acceptance or regret must be sent, and nothing but illness, accident or death should prevent us from presenting ourselves. If such obstacles intervene, immediate notice should be given the hostess, that she may supply the place at her table thus made vacant.
Do not write you will "try to come;" that you will come but your husband will not be able to do so, or in any way make your acceptance conditional. Your hostess may wish to invite another couple; she must know who will be present that she may arrange her table accordingly. Nothing is so annoying to a hostess as to be obliged to rearrange her table because of some slight excuse on the part of a guest who has once accepted,
Do not forget that an invitation to dinner is the highest social compliment, and value it accordingly; also answer at once.

Formulas for Invitations.

The formula for a dinner invitation is this:

Mr. and Mrs. John Henry Smith
request the pleasure of
Mr. and Mrs. George Brown's company at dinner,
127 Blank Avenue.
on March fifteenth at seven o'clock.

This invitation may be written on note paper or engraved on a card.

The correct form of reply is this:

Mr. and Mrs. George Brown
accept with pleasure the polite invitation of
Mr. and Mrs. John Henry Smith for dinner
on March fifteenth, at seven o'clock.

If the dinner is in honor of guests, the formula may be:

To meet
Mr. and Mrs. William Dash,
Mr. and Mrs. John Henry Smith request the pleasure of
Miss Anderson's
company at dinner,
on Wednesday, January twenty-sixth,
at seven o'clock.
R. S. V. P.     91 East Ninety-fourth street.

If the invitation must be declined, this form may be observed:
Mr. and Mrs. Brown
regret that owing to a previous engagement
they are unable to accept
Mr. and Mrs. John Henry Smith's
very kind invitation
for Tuesday evening, March fifteenth.

Any other reason, as illness, proposed absence, or the like, may be substituted for a "previous engagement."
In acknowledging invitations it is better to err on the side of over-politeness than the reverse.
If a dance or theatre party is to follow the dinner, words indicating the fact are written across the lower part of the card or in the lower left-hand corner.
"R. s v. p." stands for the French phrase, "Respondez, sit vous plait,"--meaning that a reply is desired.

The reply to an invitation should be in the same form as the invitation; thus if in the third person the reply should also be made in the third person. Such invitations are the most formal. The reply is to be addressed according to the wording of the invitation: thus if Mr. and Mrs. John Henry Smith issue it, address the reply to them; if Mrs. John Henry Smith's name alone appears, address it to her. The same rule applies to a wedding invitation. The acknowledgement is sent to the parties issuing the invitation, not to those to be married.

Must Not Ask Invitations
It is not allowable to ask for an invitation to a dinner, a luncheon or a card party for a guest or friend. These are functions arranged for a definite number of guests; to include another person is not possible. If your hostess knows you have a guest, she will, if her arrangements make it practicable, include her; if not, there is no slight to you or your guest. The presence of a guest does not excuse one from a dinner, luncheon or card party, the invitation having been already accepted. Provide some pleasure for your friend, or leave her to a quiet evening at home.
In case a guest drops out at the last moment, as sometimes happens, one may ask a very intimate friend, a relative, or some member of the family to fill the vacant seat. Such a "last minute" invitation is no compliment: one knows she is simply a substitute, but good sense and kindliness should prompt the recipient to help out in the dilemma, which may happen to her next time.

Other Particulars
Dinner invitations are issued in the name of the host and hostess, so also those for luncheons to which both men and women are invited. Invitations to teas, card and garden parties, "at homes," balls, and women's luncheons are in the name of the hostess alone.
Guests should present themselves punctually at the hour named in a dinner or luncheon invitation, allowing themselves just time to remove wraps, etc., before the meal is announced. It is almost unpardonable to be late.
Invitations are sent to people in mourning after the month following bereavement, not because acceptance is expected, but as a compliment, except that cards for dinners, luncheons and balls are not sent. Wedding cards and announcements, and cards for large general receptions are sent. During the year of mourning people thus remembered send cards with a narrow black border in acknowledgment.
Unless an entertainment is exclusively for women, an invitation to a married woman should include her husband. That he is personally unknown to the hostess does not matter.

Excerpt from the book:
Over One Thousand Tried and Tested Remedies from Mothers of the United States and Canada.