Eureka's climate is famously mild and famously foggy. The ocean keeps temperatures stable year-round — summers rarely get above 65°F (18°C) and winters are cool and wet but seldom cold. The downside is that summer fog is persistent: mornings are often completely socked in, clearing by afternoon if at all. Locals call it "sun by noon or June gloom," and the latter phrase wins more often than it should.
July to September is nominally the best weather, but fog remains common. These months also bring the most tourists, mainly people camping in the Redwood National Park corridor who pass through Eureka for supplies and a meal.
May and June are quieter and often have clearer afternoons than the peak summer months. October has some of the clearest weather of the year.
Winter brings rain, but the storms are usually not severe. It is the quietest time and a good choice for anyone who wants the Victorian streets to themselves.
The Humboldt Crabs baseball season (May–August) is a local institution — the Crabs play at Arcata Ball Park and have done so since 1945, making them one of the longest-running collegiate summer leagues in the country. Worth checking the schedule if you're in the area.