Arcata has a classic northern California coast climate — mild year-round, with heavy rainfall from November through March and dry, frequently foggy summers. Summer daytime highs rarely exceed 65°F (18°C); winter lows rarely drop below 40°F (4°C). It is never hot here in the way that inland California is hot.
Summer (June–September) is the driest period, though morning fog is common through much of June and July. This is when the North Coast is most visited. Accommodation in Arcata and Trinidad fills on weekends.
Memorial Day weekend is the single best reason to schedule your visit deliberately: the Kinetic Grand Championship runs from Arcata to Ferndale over three days, with human-powered sculpture races through beaches, marsh, and the Eel River. It is absurd and wonderful and genuinely worth the trip.
Spring and fall are good for birding at the Arcata Marsh, when shorebird migration peaks. Rainfall picks up from October and the town is quieter, though the university keeps things from going completely dead.
Winter is rainy and atmospheric — redwood forests are at their best when wet — but some coastal attractions become difficult to access on foot. A good time to visit if you don't mind being indoors part of the day.