A "biennial show" refers to an event that takes place every two years. It is a gathering or exhibition that happens once every two years. Full definition
It's a week - long biennial show that attracts millions and assures a great deal in the time to come! (auto.ndtv.com)
Portugal Arte 10 is a new biennial show of contemporary art in Lisbon. (vernissage.tv)
[1] There he organized an exhibition of George Herms, and a proposed biennial show, LAX: The Los Angeles Exhibition, a contemporary art exhibition spanning seven to eight institutions. (en.wikipedia.org)