The climate in Delhi is very peculiar.
The hottest months here are from April until June – they are considered to be a summer. At this time temperature in the city is about +42-45 degrees (Celsius). In this time there are holidays for schoolchildren and college students, though not whole 3 months but 1.5-2 months – different dates in different schools.
During the whole July, August and usually the first decade of September in Delhi lasts the rainy season, and it becomes little chillier, around +32-38. However, it does not rain here every day during rainy season, unlike in the south of India. But after rainy season the heat comes back for some time, although not such a heat like in April-May.
From about middle of November to early December and until the middle of February in Delhi lasts the season of “winter.” Local winters are quite warm, they have a temperature of +15-22 degrees during the day, but at night it can reach +4-5. But due to the lack of central heating in buildings it feels quite cold.
In the remaining months in Delhi, there is something like an off season, +27-35 degrees. There is no clear division of seasons by dates in India, Indians do consider the season change when the weather actually changes, not from some certain date.
The climate of Delhi works well for agriculture – the fruits and vegetables grow here during the whole year. But the same climate works for mosquitoes and other insects to have around quite much time of the year as well: there is no mosquitoes only in the coldest and hottest months.
Because of the 9-month hot summers, people of Delhi love and wait for the winters a lot. However, in the cities of just 300-600 km away to the north from Delhi is colder, in winters there it can even snow – because of the mountainous climate. In Delhi it is never snowing.
The winter season is more attractive and popular for tourists visiting Delhi, because in +45 degrees Celsius heat it’s difficult to visit sights, but tickets to come here are more expensive usually in winters as well.
According to me good months for tourism in Delhi are also November and March: it is not not cold and not too hot, plus the biggest Indian Festivals do happen in this time: Diwali in November and Holi in March.