Don't be Rude: RSVP

If you've been on the hosting end of a party, you know how frustrating it is when someone doesn't reply to an invitation.  Arrangements for food, whether made by the host(s) or catered, must be made in advance.  It is thoughtless not to respond to an invitation...the expected response time is three days from the receipt of the invitation.
If the invitations states:
RSVP or Please reply, you need to respond "yes" or "no" to the email or phone listed.
Regrets only means they expect you there unless you state otherwise.
If there is a reply card, you need to fill it out immediately and return it.
Sometimes there is a reply by date listed on the invitation...it is there for a reason.

It is easy to delude yourself that one guest more or less won't make a difference, but when hosting a recent event, it was five days before and I had heard from only 1/5 of the invitees. I know there are times when your availability to attend a function is in question, but either reply "no" or make the commitment to attend. You do not need to offer a reason for not attending than "I am unable to attend." If you have been traveling and come home to an invitation, call the hostess immediately and explain the situation.
Weddings present there own set of complications.  Any formal invitation requires a formal reply.  The wording may sound a bit stilted using third person, but it is the correct way to respond.  Use a white or ecru fold-over note and write on third page using black ink. Send it addressed to your hosts at the address on the back flap of the invitation envelope.
Normally, you would respond to the hostess only, but wedding responses should be addressed to the couple hosting the wedding.  Replies should be made within a week of receiving the invitation. 

Accept:

Mr. and Mrs. Alan Brown

accept with pleasure

the kind invitation of

Mr. and Mrs. Green

for

Thursday, the sixth of January

at two o'clock in the afternoon

Decline:

Mr. and Mrs. Alan Brown

regret that they are unable to accept

the very kind invitation of

Mr. and Mrs. Green

for

Thursday, the sixth of January

One or more can attend:

Mr and Mrs. Alan Brown

accept with pleasure

the kind invitation of

Mr. and Mrs. Green

for

Thursday the sixth of January

at two o'clock in the afternoon

Samantha, James and William

regret they are unable to attend

Do not use abbreviations other than titles Mr., Mrs., Ms and do not put a period at the end of any line.  Center each line on your stationery. If a person's name is not listed on the envelope of the invitation, he or she is NOT invited to the wedding.  Never ask to bring additional quests to a wedding or reception. The one variance to this rule is if you are engaged, then you may ask to bring your fiance(e).  He or she should have been invited in the first place. You'll notice that most wedding invitations today contain a reply card.  This is the hostess' attempt to illicit a response from the masses of guests who are not schooled in the art of social graces.
Until you have been stuck footing the bill for guests that failed to show, it may be hard for you to understand the waste, expense and rudeness of not replying to an invitation. There isn't a mother of a bride that I have talked to who can't forget her anger at the guests that failed to show for her daughter's wedding.  They still see the hundred dollar bills flying out the door.
The one exception to the reply rule is unsolicited invitations to business luncheons.  I feel no compunction to reply to a financial planning firm that I don't want to avail myself of their sales pitch disguised as an informative luncheon.