The important thing about an invitation is not how it is expressed, but the warmth with which it is offered. Everybody believes that -- except Miss Manners.

Miss Manners does not oppose warmth. A cold hearth is a dreadful thing, especially if you had hoped to toast marshmallows. A cold heart, when you have visions of snuggling up, is even worse.

But she believes it is also important that an invitation be given -- and received -- properly.

This is only partly because Miss Manners takes an esthetic, if not sensual, pleasure in handling thick, creamy paper covered with stilted writing. She does not expect many others to share that taste, the mere expression of which will do little to endear her to a world in which informality is considered a virtue.

Her other reason, however, is something that should make everyone fall in line behind her to demand the return of social formulas in invitations. That is that the original, spontaneous, casual invitation is totally incomprehensible to the person who receives it.

"You must come to dinner with us" is nice and warm -- but what does it mean? It may not be an invitation at all, but only a statement of genial intent. "Let's get together," "Come by and see us some evening" and "We must do something" are merely indications that an actual invitation will be made or welcomed in the future.

A genuine invitation, issued by voice or by mail, includes a date and a time: "You must come to dinner with us. How about Saturday the 7th at 7:30?"

This still leaves open such questions as:

Who is expected to attend? If the invitation is addressed to a couple, can one person go if the other is not in town? If only one of the couple is invited, is the other supposed to attend, too? May or should a single person bring a date or house guest? Are children included?

How much after the stated time is one expected to arrive? Will a full meal be served? Will it be served before everybody starves to death and/or gets drunk? If it says "buffet" and I eat beforehand, will I be sorry?

What sort of clothes should be worn? Does anybody care?

Is an answer to the invitation required, and if so, how should that be made? How long do I have to decide whether I want to go, and can I get out of it after I have accepted?

Miss Manners has lots of sympathy with the bewildered guest, provided he understands that all social invitations must be answered as quickly as possible, and that once accepted, they are binding.

Challenged on the telephone to make an immediate answer, he may stall for a day or two ("Let me check the calendar at the office," "Oh, Mary said something about our being busy that night, but I'm not sure; I'll have to ask her"), but not more. Written invitations demand a written reply in kind, regardless of whether they contain instructions, nasty little R.S.V.P. cards, or threats.

Invitations should be addressed by name to the people expected to attend, even if that means doing a little research to find out whether that awful person your friend seems to be living with has a name. Anyone who puts "and guest" or "Bring an escort" deserves what he generally gets.

Fewer people than invited may attend ("Annabelle's at a convention then, but I'd love to"), but permission must be requested for others ("Does this include the children?"; "I don't know if you've heard that Timothy and I are a couple now"; "Oh, I can't because I'll be having a house guest -- but actually, I'd love you to meet her") with the understanding that it may be politely withheld ("Oh, dear, I've already got a full table").

An invitation with an arrival time only should mean it, and not say 7 when food will be served at 10:45. Dinner invitation times should be stated and taken seriously, with guests arriving within 15 minutes, and dinner served 45 minutes later. (The word "dinner" is used to indicate that you needn't defrost anything for later; cocktail parties and receptions only promise to stave off hunger, although they can stuff you if they like.) Only parties with arrival and departure times (5 to 7) grant an hour's leeway.

Hosts cannot be coy about telling people what to wear. "Oh, anything" means even less than "We're very informal," which has no meaning at all, unless there are such strong regional or group customs that everyone invited knows "informal" means "Change into something smashing after work," or "Whatever you jog in." If the hosts don't state their terms, the guests must demand to know what they are wearing.

Traditional invitations were always rather dry, but they enabled both hosts and guests to know the terms on which the evening would be conducted. It is more fun to save spontaneous overflow of warmth and originality for the party itself.

Q. Last year, I walked into my office and found it decorated with plastic Santa Clauses, plastic garlands and a plastic tree with blinking lights. My secretary had decided to give the reception area "Christmas spirit." She had even spray-painted the windows with "snow drifts" and crooked messages saying "Merry Christmas" and "Season's Greetings."

I was appalled, but because I am a coward and didn't want to hurt her feelings (she is a nice person and paid for the decorations herself), I didn't ask her to undo what she had done, and I suffered in silence for three weeks. (She changed the window messages to "Happy New Year" after Christmas.)

Please tell me how I can say tactfully that I'd prefer no decorations or ones reflecting a different image.

A. Miss Manners cannot tell whether you object on esthetic or religious grounds. She sympathizes with you on both, but advises you not to try to deal with the former, because it is not possible to get the kitsch out of Christmas except with the most extreme Scrooge tactics.

You can gently tell your secretary that not everyone in the office is Christian, and that you are sure she did not intend, with her kindly spirit, to make anyone feel excluded. If she then alters it to represent only the season -- try to bear with it. Cleanup day is Jan. 2.