Summer in Delhi is no joke - from April the temperature climbs relentlessly to more than 45°C (113°F) in May and June and the heat doesn't really abate until October. Dust storms called loo sweep in from Rajasthan with considerable fury. The arrival of the monsoon, at the end of June, brings intense humidity - the murder rate usually peaks in this month.
From November to March is the best time to visit, with cool but sunny weather. Also, Holi, one of the most exuberant Hindu festivals, takes place around this time, in February or March. October sees the end of the monsoon, but is reasonably pleasant, although the days can still be pretty hot. Nights can be quite chilly in December and January.
Weather
One of Delhi's drawbacks is that for more than half the year the climate is lousy. For about four months - from November until about March - it's fine, with daytime temperatures between 21 and 30°C (70-86°F). However, by the end of April it's hot, while May and June are really hot - temperatures can swoop up to more than 45°C (113°F). In July it gets cooler but the humidity really hits, often with all-day rain through August and September.