The best season for going to India is winter, which is from November to March. But India is a gigantic country so there are considerable differences in climate between the regions. In December and January, for example, it can get very cold in the north. The south-east and the south often experience heavy rain from October to the beginning of December. So you need to adapt your departure dates according to the region you intend to visit, and, in any case, avoid going during the monsoon, which gradually extends over all the country from the end of May until October.