The single most important thing in Venice is your shoes. The entire city is walked — no cars, no buses, no bicycles — on uneven stone, arched bridges with steps, and occasionally waterlogged paths. Comfortable, supportive walking shoes with decent grip are non-negotiable.
Summer (June to August) is hot and humid — 28 to 35 degrees. Light, breathable clothing, sun hat and sunscreen are essential. For churches, cover shoulders and knees — a light scarf or cardigan handles this.
Spring (March to May) and autumn (September to November) are the sweet spot. Layering is key. Autumn brings the acqua alta (high water) risk — waterproof shoes or packable rubber overshoes (available cheaply in the city) are genuinely useful.
Winter (December to February) is cold and damp — 0 to 8 degrees with fog off the lagoon. Warm waterproof coat, layers, gloves and hat. Carnival (February) brings masks, costumes and celebratory mood — dress warmly underneath.