Basic practice happens every Sunday except when there is a major event that runs on Sunday.
From the South on Highway 29: Take the Imola Ave. exit to the East; turn left on Jefferson, and continue until you reach the park, on your right. From the North on Highway 29: Take the First Street exit toward Downtown Napa. Turn right on First Street, and cross the Highway. Turn left on Second Street and right on Jefferson; the park will be on your left in a couple of blocks.
Events are usually located on the Jefferson Street side of the park near the intersection with Stockton Street.