Santa Cruz has a famously temperate climate, but June and July can be surprisingly foggy — the same marine layer that keeps the Bay Area cool drifts south along the coast and can make summer mornings grey and cold. The locals call it "Junuary." Late August through October is typically the best period: the fog has burned off, the water is as warm as it gets (still cold by most standards, around 15–18°C), and the summer tourist crowd has thinned.
The Woodies on the Wharf car show in late June is a Santa Cruz tradition — vintage woodie wagons lined up on the pier, a spectacle that is either charming or campy depending on your tolerance for surf nostalgia. The Shakespeare Santa Cruz festival (July–August) at UCSC is a long-standing tradition that occupies an outdoor theatre in the redwoods.
Winter is quiet but mild by California standards. The surf at Steamer Lane peaks in winter when North Pacific swells arrive, and the monarch butterfly colony at Natural Bridges is at its peak from October through February. A winter weekend in Santa Cruz, when the Boardwalk is quiet and the coast is yours, is more pleasurable than the summer crowds suggest.