The Venice Biennale refers to an arts organization based in Venice and the name of the original and principal biennial exhibition the organization organizes. There are several separate events, that held every year, but not all-in-one.
For instance, The Art Biennale, a contemporary visual art exhibition and so called because it is held biennially (in odd-numbered years), is the original biennale on which others in the world have been modeled.
So, there is often an opportunity to drop in if you are in Venice to visit it.
Web site of Venice Bienalle http://www.labiennale.org/en