An announcement setting is when the defense lawyer, the accused, and the assistant district attorney meet in court to discuss the case. It will be determined during this time whether the case will be dismissed or plea bargained or set for trial. There can be more than one "announcement" set, and the person accused must appear in court each time. The first announcement is generally when the person accused of a crime is asked whether or not he/she wishes to reach a plea agreement with the district attorney.
The letter received means that the person must appear in court at that time to see what is going to happen with his/her case.