Mill Valley in December

April through October gives you the best weather in Mill Valley. Summer means no rain but also means fog that rolls in most afternoons — pack layers even in July. May and September hit the sweet spot with warm days and clear skies. The Mill Valley Film Festival happens in October, bringing crowds and higher hotel rates. Winter brings rain that can last for days, but also fewer tourists and green hills that photographers love. Spring wildflowers bloom best in March and April along the Dipsea Trail. Avoid Memorial Day and Labor Day weekends unless you enjoy fighting for parking spots.