A mission reunion is an event where returned missionaries (those who have gone on missions and come home again) get together. The reunions are usually with those who served in the same mission under the same mission president.
Mission reunions are usually held somewhere in Utah on the Friday before General Conference in October and April. Many of them are held in LDS meetinghouses with dinners or refreshments. The main entertainment is visiting and catching up with people. Most of the time, returned missionaries are encouraged to bring spouses and families so that everyone can meet them.
Mission reunions are usually posted online on a variety of sites devoted to the mission. The Deseret News also posts reunion information such as times and location on this website: LDS Missionary Reunion. The BYU Daily Universe also gives times and locations for reunions.