San Marino has a Mediterranean-influenced climate similar to the surrounding Italian Romagna region, with warm summers and mild winters.
Summer (June-August) is the busiest season. Temperatures reach 25-30C and the views from Monte Titano are at their best on clear days. Weekends can be very crowded with day-trippers from the Italian coast. Visit on a weekday if you can.
Spring (April-May) and autumn (September-October) are ideal. The weather is pleasant, crowds are manageable, and the light is beautiful for views across the Apennines and toward the Adriatic.
Winter (November-March) is quiet. Some smaller museums may have reduced hours, and fog can obscure the famous views. But you will have the place largely to yourself, and on clear winter days the panoramas are stunning.
The medieval festival Giornate Medievali takes place in July, with costumed parades, flag-throwing, and crossbow tournaments. San Marino's national day on September 3rd is celebrated with ceremonies and fireworks.