Spring (March to May) is the best time for Galveston — warm but not oppressively hot, the beaches are uncrowded, and the historic district is at its most pleasant. Mardi Gras (February or March) is Galveston's signature event, a surprisingly large celebration that draws hundreds of thousands of people and is one of the largest Mardi Gras events in the country after New Orleans. Hotel rooms need to be booked months ahead.
Summer (June through August) is hot and humid — temperatures regularly reach the 90s Fahrenheit with high humidity — and the beaches are packed with Houston families. The water is warm enough for swimming from May through October. Hurricane season runs June through November, with August and September the peak months; Galveston has been struck multiple times and the threat is real, though most storms track elsewhere. Check forecasts before travelling in late summer.
Fall (September to November) is underrated — the water is still warm, the summer crowds have thinned, and the island is more relaxed. Winter is mild by national standards (rarely below 40°F) and the historic district is perfectly pleasant to walk around, though many Seawall establishments reduce hours or close.