Castelsardo is busiest in July and August when it draws beach visitors from across Sardinia and the Italian mainland. The old town gets crowded, parking is difficult, and accommodation is expensive. May, June, and September are much more pleasant — warm enough for swimming, quiet enough to walk the castle lanes in peace.
Lunissanti is the town's most atmospheric annual event: a Holy Week procession held over the three days before Easter. Hooded penitents carry sacred images through the narrow streets of the old town, ending with a torchlight procession on Easter Sunday evening. The event draws visitors from across the Anglona region and is genuinely affecting — the combination of the medieval setting, the darkness, the candlelight, and the old religious music makes for an experience unlike any other in Sardinia. If you can time a visit to coincide with Holy Week, do.