Chillai Kalan is the traditional term for 40 days of hard winter in Kashmir. Every year, it is the harshest portion of winter, lasting from December 21 to January 31. Chillai-Kalan is followed by a 20-day Chillai Khurd from January 30 and February 18 and a 10-day Chillai Bachha between February 19 and February 28.

During these 40 days in Kashmir, the nights are cold and the days are hot, with temperatures in the single digits. During Chillai-Kalan, the weather in Kashmir's valley remains chilly, with the average temperature staying around freezing. Snow that falls at this time of year settles and lasts much longer. This snow contributes to the Valley's glaciers and supplies the perennial reservoirs that feed Kashmir's waterways, rivers, and ponds throughout the summer season. Any snowfall that occurs after the chillai Kalan lasts only a few days.