The Castillo de San Marcos is open to the public every day of the year except Thanksgiving Day (fourth Thursday in November) and Christmas Day (December 25).
First admission is at 8:45 am and last admission is at 5:00 pm with the Castillo closing at 5:15 pm. The park grounds are closed from midnight until 5:30 am.
Visitation is lowest from the middle of September through late November.
Visitation is highest June through Labor Day, and quite high on summer and holiday weekends when an average of 3500 people visit the Castillo daily.
School group visitation is quite high February through May. High visitation can also be expected during the December holidays.