Southern California's climate makes Buena Park pleasant year-round, with warm, dry summers and mild winters. The main consideration is crowd levels at Knott's Berry Farm: summer school holidays and weekends from June through August are the busiest and most expensive times to visit. Weekdays in spring and autumn offer shorter queue times and better prices.
Knott's Soak City waterpark is only open from late May through early September, so if that is part of your plan, summer is the right window. Halloween Haunt, the park's annual October fright event, draws large crowds but is genuinely well-produced and worth timing a visit around if you enjoy haunted attractions.