Cumberland Island is accessible year-round, but the experience varies considerably by season.
Spring (March to May) is the best all-round time. Temperatures are pleasant (70–80°F), the vegetation is green, wildflowers are blooming, and the island is less crowded than summer. Migratory birds pass through in good numbers.
Summer (June to August) is hot and humid — temperatures regularly exceed 90°F and the mosquitoes and biting flies (particularly the notorious "no-see-ums") can be ferocious. Loggerhead turtle nesting activity peaks in June and July. The island is at its busiest in summer; the daily visitor limit means the ferry books out quickly.
Autumn (September to November) is excellent. Temperatures moderate, insects become less oppressive, and the forest takes on subtle colour. Loggerhead hatchlings emerge in September and October. Crowds thin noticeably after Labor Day.
Winter (December to February) is the quietest period. Temperatures are mild (50–65°F) by most standards, the island is essentially to yourself, and wildlife is active — particularly shore and wading birds. An ideal time for anyone who wants the place in near-solitude.
Practical notes: The ferry from St. Marys runs daily except Tuesdays and Wednesdays. The daily visitor limit is 300 people. Reservations are essential in summer and advisable year-round — book via recreation.gov.