Six Flags Discovery Kingdom in Vallejo has the lightest attendance in the months of April, May, September and October and on weekdays in the summer months. Visitors looking for a hotel room in the area may find them to be less available and more expensive in September and October, however.
The primary visitor demographic of Six Flags is families with children, so in general the park becomes less busy when kids are in school. Winters in the area tend to be relatively mild and without snow, so the park can still be crowded on the weekends even in these months. Weekdays usually see lighter attendance year-round, even in the summer as parents still have to work even when the kids are out of school.
The Six Flags website recommends the spring and fall months as the least busy, even on weekends. However, Vallejo is close to the Napa Valley, which sees significant wine tourism that is typically at its peak during the harvest months of September and October. Wine tourism generally has little impact on Six Flags attendance, but hotels in the area may be hard to come by or more expensive than usual during these months, particularly on weekends.