Santa Clara has a mild Mediterranean climate year-round. Summer (June through September) is warm and dry, with temperatures typically between 25–30°C and almost no rain. This is the best time for California's Great America and outdoor events. Winter (December through February) is cool and rainy but rarely cold enough to disrupt plans — frost is almost unheard of.
49ers home games at Levi's Stadium run September through January and make for a lively weekend in the city. The area around the stadium fills up hours before kickoff.
The Gilroy Garlic Festival (late July, about 30 km south in Gilroy) draws enormous crowds for three days of garlic-themed food, live music, and the famous garlic ice cream. It sounds gimmicky; it is genuinely fun and worth the detour if you are in the area in summer.