Penang has the wet tropical climate of most of peninsular Malaysia — hot and humid all year (highs 30-33°C, lows 23-25°C), with two wetter periods rather than a true dry-wet division. The driest stretch is January to March: still warm, with the lowest humidity and the smallest chance of rain. April through May see the first inter-monsoon thunderstorms. June to August is the southwest monsoon period — bright sunshine in the morning, heavy short downpours most afternoons. September to November is the wettest spell, with longer rains driven by the inter-monsoon. December is variable, often pleasant.
Chinese New Year (late January or February) is the major festival of the year: Kek Lok Si is decked in tens of thousands of lights, the clan houses host elaborate celebrations, and street food stalls cook overtime. The downside is that many small businesses close for several days and rates spike. Thaipusam (late January or February) is the spectacular Hindu pilgrimage to the Murugan temple at Waterfall Hill, with chariot processions and self-mortification — worth timing a visit around if you can.
The annual George Town Festival in August features performance, exhibitions and street events across the heritage zone — the festival commissioned the original street art in 2012.
For the best weather and the smallest crowds, aim for February to April outside Chinese New Year, or the brief shoulder in September.