The best times to visit Messina are April through June and September through October, when the heat is manageable, the crowds are modest, and the strait is at its most navigable. July and August are hot, busy with ferry traffic, and the city empties somewhat as Messinesi head to the beaches. The regional museum and the cathedral are worth visiting in any weather, but the outdoor sights and the waterfront are more pleasant in the shoulder seasons.
The Feast of the Madonna della Lettera on June 3rd is Messina's main civic festival, commemorating the patron saint with a waterfront procession and celebrations. The Vara procession in August is one of Sicily's most spectacular religious pageants — a towering 50-foot mechanical float carrying dozens of figures is hauled through the streets by hundreds of people, a tradition dating to 1253. If your visit coincides with either, count it as good luck. Winter in Messina is mild and rainy; the city functions normally and tourist sites are quiet, making it a reasonable low-season choice for anyone who doesn't need beach weather.