Picnic Day is an annual open house event held in April at the University of California, Davis. Picnic Day was first held on May 22, 1909. It has grown to be what is believed to be the largest student-run event in the United States, typically drawing more than 50,000 visitors. In 2009, around 125,000 visitors attended Picnic Day – a new attendance record.

The University of California, Davis is a public land-grant research university in Davis, California, United States. It is the northernmost of the ten campuses of the University of California system. The institution was first founded as an agricultural branch of the system in 1905, known as University Farm, and became the seventh campus of the University of California in 1959.