Best Time to Visit San Juan Island

San Juan Island National Historical Park is a National Historical Park in WA and a moderately visited park, drawing about 421,000 recreation visits in 2025. Visitation has climbed over the past decade, up roughly 68.5%, so crowds are trending heavier each year. Crowds concentrate sharply by season: July is the busiest month at about 1.8× the park's average month, while January is the calmest at roughly 0.5× — a 4-to-1 swing between high and low season. Roughly 42% of the year's visitors arrive in the three summer months of June, July and August alone, so timing a trip outside that window is the single biggest lever on how crowded the park feels. Weather swings from an average high-season warmth near 62°F in August down to about 42°F in December, with the wettest stretch in November. About 6 months a year see comfortable average temperatures in the 50–80°F range, which is part of why crowd timing and weather timing don't always line up. The sweet spot is April: best overall balance of crowds, weather and daylight. Visit then and you trade a little peak-season certainty for noticeably shorter lines and easier parking than the July rush. Daylight ranges from about 8.1 hours in midwinter to 15.9 hours near the summer solstice, which shapes how much ground you can cover in a day. These crowd figures are drawn from 1979–2025 of National Park Service visitor-use records, paired with 1991–2020 NOAA climate normals, and are updated as new monthly data is released.