Social gatherings are usually informal in the US.
Most people do not use parties to show wealth or taste.
Parties are opportunities for people to gather, talk, and relax.
The host-guest relationship is very informal because your host wants you to "feel at home." This means you may have to serve yourself.
Invitations are usually informal, but tell the time and place.
If someone says "come and see me sometime" or "drop in," this is not an invitation. It means that you must telephone before you drop in.
If you accept an invitation, it is important that you go and be on time. The host expects you to call as soon as possible if you must cancel. It is considered rude to simply not go when you said you would. Refusing an invitation is not considered rude in the United States.
A formal, written invitation, requires a response and often a written response.
* If the invitation says "RSVP," you must call the host and tell him whether or not you will be there. If you cannot attend or do not plan to attend, say so. Do not say "yes" just to be polite. The "RSVP" is to help the host plan how much food and drink to prepare; an accurate count is necessary.
* If your invitation says, "Regrets only," you only need to tell your host if you do not plan to attend. If you are unsure how to dress, call the host and ask, "What should I wear?"
* Student gatherings are much less formal. What time you arrive makes little difference. Many student parties are "BYOB" (bring you own beverage). You will make enemies rapidly if you go to parties and drink other people's beverages. If a party is BYOB, then you should BYOB!
Gifts for Hosts: It is not necessary to bring flowers, candy or a gift to a dinner host. However, it is a nice thing to do. You should always say "thank you" when you leave. It is also polite to write a note or telephone the next day and repeat your thank you. Overnight house guests usually bring a small, inexpensive gift to the host, such as a souvenir from your home country or other small remembrance.
Table Etiquette: If you have dietary restrictions, it is a good idea to tell your host when you are invited. When you are at the dinner table and are asked if you would like something, do not refuse out of politeness. The host or hostess probably won't ask you a second time. After you have had enough, it is all right to politely decline additional servings.
When filling your plate at a buffet, it is polite to take one serving of each item only. After everyone has been served, it is fine to return for “seconds” (more food).