October to March is the most comfortable time to visit Mangalore. The monsoon arrives in June and departs in September, and during those months the city receives some of the heaviest rainfall on the Indian coast — roads flood, the beach is rough, and the humidity is extreme. The upside is that the Western Ghats behind the city turn intensely green and the rivers run high.
November to February is ideal: dry, relatively cool (around 25–30°C), and clear. This is also when the major festivals fall — Navratri in October is spectacular at Kudroli Temple. April and May are very hot (up to 38°C) but manageable if you plan outdoor activities for early morning and rest in the middle of the day.