Seminars are usually made over the weekend but it is not a rule. Summer camps are also popular. They are lead by Aikido masters. Very often these are Japanese instructors but there are others too. Some people travel to other cities and even countries to take part in the seminars. They combine Aikido practice with traveling, meeting new people and making new friendships.
They are very important for one's progress and development of Aikido techniques. On everyday training, people are usually accustomed to each other. That's why seminars are so great. There, you have the opportunity to practice with people you don't meet each day, and the people with whom you have not practiced before.
Very often the techniques shown in the seminar are different from techniques that are practiced in the daily training. But that is the point of the seminar - learning new things and meeting new people. Sometimes (especially younger students) find it difficult to repeat the demonstrated technique, but it is a way to learn and progress and the results will show in the coming months.