Most invitations are made by telephone or in person.Occasionally,for a formal party or dinner,an invitation requires an answer,either in writing or by telephone.If you later discover that you cannot attend,you should telephone the host or hostess and explain why you cannot come.You should do this before the party.
You should arrive at the time you werer invited for,or within five or ten minutes after that time.In the United States,whether for business or pleasure,people are very aware of the time and as a result are usually on time for business meeting and social activities.