Held every year during the fall (typically October/November), it's a special open school event in which students display their school work, especially any artistic achievements, throughout different rooms in the school. This is a great opportunity for parents/guardians to see what their children have been working on, as well as for former teachers to say hello and see how much their former pupils have matured. In junior high school, students also sing in chorus with their grade, the brass band club performs various musical pieces, and the student council makes a special presentation/performance.
In high school, each class runs their own booth or activity (such as cafes, haunted houses, plays, game shows, etc.) from start to finish, planning and making materials with little teacher intervention. Extracurricular clubs also run a booth/activity. The event is typically held on a Saturday for junior high schools, and is an official work day. ALTs are usually expected to attend (sometimes even perform, although that is rare), and they should be offered compensatory leave (daikyuu) on the following Monday. (If that is not possible due to scheduliing conflicts with ES, ALTs can negotiate a different day to take daikyuu.) If you are a high school ALT, please ask your school for more information about scheduling. At elementary schools, they hold a similar event called Ongakkai.